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Thursday, 21 January 2010
Thursday, 14 January 2010
My blog on steps required to convert logical data model to physical data model
Hi All,
Please find the URL of my blog which describes the steps to convert the Logical Data Model into Physical Data Model.
http://ldmtopdm.blogspot.com/
Regards,
Sandeep
Please find the URL of my blog which describes the steps to convert the Logical Data Model into Physical Data Model.
http://ldmtopdm.blogspot.com/
Regards,
Sandeep
Tuesday, 12 January 2010
Role and responsibilities of Data Architect/Integration Architect
Profile as Data Architect/Integration Architect
•Understanding the customer’s business requirements, business processes and system functionality to develop the data design and maintain the source of data.
•Responsible for developing and maintaining a formal description of the data and data structures - this can include data definitions, data models [Conceptual, Logical and Physical Data Model, data dictionaries, data flow diagrams, etc. (in short metadata). Data architecture includes topics such as metadata management, business semantics, data modeling and metadata workflow management.
•The architecture covers databases, data integration and the means to get to the data
•Resolve the end to end data issues, design issues and end to end test issues with the help of data dictionaries.
•Provides the means of system integration by understanding the source and target data and build the data mapping.
•High-level data architecture documentation, which illustrates the source and destination
of data to and/or from approved data sources
•Development of a data strategy and design specifications
•Design strategy and design for managing history data
•Selection of capabilities and systems to meet business information needs
•Evaluating new techniques for enhanced functionality
•Understanding the customer’s business requirements, business processes and system functionality to develop the data design and maintain the source of data.
•Responsible for developing and maintaining a formal description of the data and data structures - this can include data definitions, data models [Conceptual, Logical and Physical Data Model, data dictionaries, data flow diagrams, etc. (in short metadata). Data architecture includes topics such as metadata management, business semantics, data modeling and metadata workflow management.
•The architecture covers databases, data integration and the means to get to the data
•Resolve the end to end data issues, design issues and end to end test issues with the help of data dictionaries.
•Provides the means of system integration by understanding the source and target data and build the data mapping.
•High-level data architecture documentation, which illustrates the source and destination
of data to and/or from approved data sources
•Development of a data strategy and design specifications
•Design strategy and design for managing history data
•Selection of capabilities and systems to meet business information needs
•Evaluating new techniques for enhanced functionality
Friday, 8 January 2010
SQL PL SQL Oracle Database Questions Answers
EMP Table :
EMPNO ENAME JOB MGR HIREDATE SAL COMM DEPTNO
7369 SMITH CLERK 7902 17-DEC-80 800 20
7499 ALLEN SALESMAN7698 20-FEB-81 1600 300 30
7521 WARD SALESMAN7698 22-FEB-81 1250 500 30
7566 JONES MANAGER 7839 02-APR-81 2975 20
7654 MARTIN SALESMAN7698 28-SEP-81 1250 1400 30
7698 BLAKE MANAGER 7839 01-MAY-81 2850 30
7782 CLARK MANAGER 7839 09-JUN-81 2450 10
7788 SCOTT ANALYST 7566 19-APR-87 3000 20
7839 KING PRESIDENT 17-NOV-81 5000 10
7844 TURNER SALESMAN7698 08-SEP-81 1500 0 30
7876 ADAMS CLERK 7788 23-MAY-87 1100 20
7900 JAMES CLERK 7698 03-DEC-81 950 30
7902 FORD ANALYST 7566 03-DEC-81 3000 20
7934 MILLER CLERK 7782 23-JAN-82 1300 10
Dept :
DEPTNO DNAME LOC
10 ACCOUNTING NEW YORK
20 RESEARCH DALLAS
30 SALES CHICAGO
40 OPERATIONS BOSTON
SALGrade:
GRADE LOSAL HISAL
1 700 1200
2 1201 1400
3 1401 2000
4 2001 3000
5 3001 9999
Q)How do I eliminate the duplicate rows ?
delete from table_name where rowid not in (select max(rowid) from table group by duplicate_values_field_name);
select max(rowid) from scott.emp group by job
Looks like 5 rows will remain after the deletions
MAX(ROWID)
AAAMgHAAEAAAAAeAAG
AAAMgHAAEAAAAAeAAM
AAAMgHAAEAAAAAeAAI
AAAMgHAAEAAAAAeAAJ
AAAMgHAAEAAAAAeAAN
Then get rid of the remaining duplicate rows.
Delete from emp where rowed not in(select max(rowid) from scott.emp group by job )
Q)How to identify the duplicate rows ?
select job, count(*) from scott.emp group by job having count(*)>1
JOB COUNT(*)
MANAGER 3
ANALYST 2
SALESMAN4
CLERK 4
Q)How do I display row number with records?
To achive this use rownum pseudocolumn with query, like SQL> SQL> select rownum, ename from emp;
Output:
1 Scott
2 Millor
3 Jiyo
4 Smith
Q)if commission is null then the text “Not Applicable” want to display, instead of blank space. How do I write the query?
select nvl(to_char(comm),'NA') from scott.emp;
NVL(TO_CHAR(COMM),'NA')
NA
300
500
NA
1400
NA
NA
NA
NA
0
NA
NA
NA
NA
Q)Display the records between two range
select rownum, empno, ename from scott.emp where rowid in
(select rowid from scott.emp where rownum <=14
minus
select rowid from scott.emp where rownum<7);
ROWNUM EMPNO ENAME
1 7782 CLARK
2 7788 SCOTT
3 7839 KING
4 7844 TURNER
5 7876 ADAMS
6 7900 JAMES
7 7902 FORD
8 7934 MILLER
Q)Oracle cursor : Implicit & Explicit cursors
Oracle uses work areas called private SQL areas to create SQL statements.
PL/SQL construct to identify each and every work are used, is called as Cursor.
For SQL queries returning a single row, PL/SQL declares all implicit cursors.
For queries that returning more than one row, the cursor needs to be explicitly declared.
Q)Explicit Cursor attributes
There are four cursor attributes used in Oracle
cursor_name%Found, cursor_name%NOTFOUND, cursor_name%ROWCOUNT, cursor_name%ISOPEN
Q)Implicit Cursor attributes
Same as explicit cursor but prefixed by the word SQL
SQL%Found, SQL%NOTFOUND, SQL%ROWCOUNT, SQL%ISOPEN
Q)What is the maximum number of triggers, can apply to a single table?
12 triggers
Q)What are PL/SQL Cursor Exceptions?
Cursor_Already_Open, Invalid_Cursor
Q)Any three PL/SQL Exceptions?
Too_many_rows, No_Data_Found, Value_Error, Zero_Error, Others
Q)Display Odd/ Even number of records
Odd number of records:
select * from emp where (rowid,1) in (select rowid, mod(rownum,2) from emp);
Even number of records:
select * from emp where (rowid,0) in (select rowid, mod(rownum,2) from emp)
Q)Find out nth highest salary from emp table
SELECT DISTINCT (a.sal) FROM EMP A WHERE &N = (SELECT COUNT (DISTINCT (b.sal)) FROM EMP B WHERE a.sal<=b.sal);
For explanation let a table emp with sal column like below
sal
1200
1300
1500
1200
1250
1700
1250
2000
See DISTINCT word is there in the query
So, you have to find the distinct sal first.
sal
1200
1300
1500
1250
1700
2000
Now see the condition a.sal<= b.sal
This condition compares a.sal and b.sal. The COUNT counts
how many times the a.sal is less than or equal to b.sal,
and gives this value as the output of sub query.
(N.B. comparing to same value means EQUAL SO count is 1).
This count is the value of N.
So after comparision the value of N for different salaries
are like
sal N
1200 6
1300 4
1500 3
1250 5
1700 2
2000 1
Now on querying when you give a value of N the
corresponding value of salary is selected.
Like if you are giving N=2 , then 1700 is displayed.
Q)What is correlated sub-query ?
Correlated sub query is a sub query which has reference to the main query
Q)What is a Non- corelated subquery
Non corelated subquery, where the subquery has been executed once for the entire parent statement.
Q)What are the usage of SAVEPOINTS ?
SAVEPOINTS are used to subdivide a transaction into smaller parts. It enables rolling back part of a transaction. Maximum of five save points are allowed.
Q)What is meant by Scrollable cursor
A scrollable cursor, however, can move forward and backward, and can seek any desired record in the cursor. Such operations are common in applications that present results sets in scrolling windows. With a scrollable cursor, application developers do not need to create and manage their own buffer for the records.
Q)What are the more common pseudo-columns
SELECT rownum, rowid FROM sometable
pseudo column is a column that is not present in the database.
it is used for unique identification of rows like
ROWNUM,ROWID,LEVEL,NEXTVAL,CURVAL,NULL etc.
Q)Where the integrity constraints are stored in Data Dictionary ?
The integrity constraints are stored in USER_CONSTRAINTS.
Q)What is ON DELETE CASCADE ?
When ON DELETE CASCADE is specified ORACLE maintains referential integrity by automatically removing dependent foreign key values if a referenced primary or unique key value is removed.
Q)What is a join ? Explain the different types of joins ?
A join is used to combine rows from multiple tables. A join is performed whenever two or more tables is listed in the FROM clause of an SQL statement.
The types of joins that you learn are
EQUIJOINS
NATURAL JOINS
NON-EQUIJOINS
OUTER JOINS
SELF JOINS
Equijoin : Perhaps the most used and important of the joins is the EQUIJOIN, also referred to as an INNER JOIN. The EQUIJOIN joins two tables with a common column in which each is usually the primary key.
SELECT EMPLOYEE_TBL.EMP_ID,
EMPLOYEE_PAY_TBL.DATE_HIRE
FROM EMPLOYEE_TBL,
EMPLOYEE_PAY_TBL
WHERE EMPLOYEE_TBL.EMP_ID = EMPLOYEE_PAY_TBL.EMP_ID;
Natural Join :
A natural join is a join statement that compares the common columns of both tables with each other. One should check whether common columns exist in both tables before doing a natural join.
Natural joins may cause problems if columns are added or renamed. Also, no more than two tables can be joined using this method. So, it is best to avoid natural joins as far as possible.
Examples
This is the same as an equi join on (emp.deptno = dept.deptno):
SELECT dname, ename FROM dept NATURAL JOIN emp
Same query on the HR sample schema:
SELECT department_name, first_name||' '||last_name
FROM departments NATURAL JOIN employees;
Nonequi join:
An nonequi (or theta) join is a join statement that uses an unequal operation (i.e: <>, >, <, !=, BETWEEN, etc.) to match rows from different tables. The converse of an nonequi join is a equi join operation.
Using Oracle join syntax:
SELECT e.ename, e.sal, s.grade
FROM emp e, salgrade s
WHERE e.sal BETWEEN s.losal and s.hisal;
Using ANSI join syntax:
SELECT e.ename, e.sal, s.grade
FROM emp e INNER JOIN salgrade s
ON e.sal BETWEEN s.losal and s.hisal;
Self join:
A self join is a join in which a table is joined with itself. For example, when you require details about an employee and his manager (also an employee).
Oracle join syntax:
SELECT e1.ename||' works for '||e2.ename "Employees and their Managers"
FROM emp e1, emp e2
WHERE e1.mgr = e2.empno;
ANSI join syntax (Oracle 9i and above):
SELECT e1.ename||' works for '||e2.ename "Employees and their Managers"
FROM emp e1
JOIN emp e2
ON (e1.mgr = e2.empno);
Outer Joins :
An outer join extends the result of a simple join. An outer join returns all rows that satisfy the join condition and also returns some or all of those rows from one table for which no rows from the other satisfy the join condition.
To write a query that performs an outer join of tables A and B and returns all rows from A (a left outer join), use the ANSI LEFT [OUTER] JOIN syntax, or apply the outer join operator (+) to all columns of B in the join condition. For all rows in A that have no matching rows in B, Oracle returns null for any select list expressions containing columns of B.
To write a query that performs an outer join of tables A and B and returns all rows from B (a right outer join), use the ANSI RIGHT [OUTER] syntax, or apply the outer join operator (+) to all columns of A in the join condition. For all rows in B that have no matching rows in A, Oracle returns null for any select list expressions containing columns of A.
To write a query that performs an outer join and and returns all rows from A and B, extended with nulls if they do not satisfy the join condition (a full outer join), use the ANSI FULL [OUTER] JOIN syntax.
For example the following query returns all the employees and department names and even those department names where no employee is working.
select e.empno,e.ename,e.sal,e.deptno,d.dname,d.city from emp e, dept d where e.deptno(+)=d.deptno;
That is specify the (+) sign to the column which is lacking values.
Cartesian Products :
If two tables in a join query have no join condition, Oracle returns their Cartesian product. Oracle combines each row of one table with each row of the other. A Cartesian product always generates many rows and is rarely useful. For example, the Cartesian product of two tables, each with 100 rows, has 10,000 rows. Always include a join condition unless you specifically need a Cartesian product
Q)What is the Subquery ?
Sub query is a query whose return values are used in filtering conditions of the main query. Subqueries will be executed once for the entire parent statement.
Q)Can a view be updated/inserted/deleted? If Yes under what conditions ?
A View can be updated/deleted/inserted if it has only one base table if the view is based on columns from one or more tables then insert, update and delete is not possible.
Q)How many LONG columns are allowed in a table ? Is it possible to use LONG columns in WHERE clause or ORDER BY ?
Only one LONG columns is allowed. It is not possible to use LONG column in WHERE or ORDER BY clause.
Q)What is Referential Integrity ?
Maintaining data integrity through a set of rules that restrict the values of one or more columns of the tables based on the values of primary key or unique key of the referenced table.
Q)What is ROWID ?
ROWID is a pseudo column attached to each row of a table. It is 18 character long, blockno, rownumber are the components of ROWID.
Q)Drop the column
There are 2 ways of doing it :
alter table"table name" drop column "column name";
This drops the column immediately. However if there is huge data and you would like to postpone the task of dropping the columns you can make the columns unused and drop the unused columns during the weekend or less peak activity time.
Q)UNION,INTERSECT,MINUS,UNION ALL ?
Then, a few values are inserted:
alter session set nls_date_format='dd.mm.yyyy';
insert into table_1 values ( 3, 'hello' , to_date('28.08.1970'));
insert into table_1 values ( 42, 'galaxy', to_date('01.01.2001'));
insert into table_1 values (100, 'bye' , to_date('09.02.2004'));
insert into table_2 values ( 3, 'bye' , to_date('28.08.1970'));
insert into table_2 values ( 42, 'galaxy', to_date('01.01.2001'));
insert into table_2 values ( 60, 'bye' , to_date('09.02.2004'));
insert into table_2 values ( 3, 'hello' , to_date('05.05.2002'));
union all:
union all selects all rows from all select statements:
select col_1, col_2, col_3 from table_1 union all
select col_1, col_2, col_3 from table_2;
As can be seen, all records of both tables are returned:
COL_1 COL_2 COL_3
---------- ---------- ----------
3 hello 28.08.1970
42 galaxy 01.01.2001
100 bye 09.02.2004
3 bye 28.08.1970
42 galaxy 01.01.2001
60 bye 09.02.2004
3 hello 05.05.2002
union :
union all is very similar to union, however, it dismisses duplicate rows found across different select statements:
select col_1, col_2, col_3 from table_1 union
select col_1, col_2, col_3 from table_2;
The galaxy record is a duplicate. Hence, it is returned only once:
COL_1 COL_2 COL_3
---------- ---------- ----------
3 bye 28.08.1970
3 hello 28.08.1970
3 hello 05.05.2002
42 galaxy 01.01.2001
60 bye 09.02.2004
100 bye 09.02.2004
intersect:
intersect only returns the rows that are found in all select statements:
select col_1, col_2, col_3 from table_1 intersect
select col_1, col_2, col_3 from table_2;
Only the galaxy record is returned. It's the only record that is stored in both tables:
COL_1 COL_2 COL_3
---------- ---------- ----------
42 galaxy 01.01.2001
minus:
minus returns all rows from the first select statements except those who are duplicated in a following select statement:
select col_1, col_2, col_3 from table_1 minus
select col_1, col_2, col_3 from table_2;
As the galaxy record is found in both tables, it is removed from the first table's record set:
COL_1 COL_2 COL_3
---------- ---------- ----------
3 hello 28.08.1970
100 bye 09.02.2004
The following query is valid:
SELECT 3 FROM DUAL
INTERSECT
SELECT 3f FROM DUAL;
This is implicitly converted to the following compound query:
SELECT TO_BINARY_FLOAT(3) FROM DUAL
INTERSECT
SELECT 3f FROM DUAL;
The following query returns an error:
SELECT '3' FROM DUAL
INTERSECT
SELECT 3f FROM DUAL;
Q)What is CYCLE/NO CYCLE in a Sequence ?
CYCLE specifies that the sequence continues to generate values after reaching either maximum or minimum value. After pan ascending sequence reaches its maximum value, it generates its minimum value. After a descending sequence reaches its minimum, it generates its maximum.NO CYCLE specifies that the sequence cannot generate more values after reaching its maximum or minimum value.
Q)How will you a activate/deactivate integrity constraints ?
The integrity constraints can be enabled or disabled by ALTER TABLE ENABLE constraint/DISABLE constraint.
Q)What is difference between CHAR and VARCHAR2 ? What is the maximum SIZE allowed for each type ?
CHAR pads blank spaces to the maximum length. VARCHAR2 does not pad blank spaces. For CHAR it is 255 and 2000 for VARCHAR2.
Q)Database link
Database Link is a named path through which a remote database can be accessed.
Q)What are the data types allowed in a table ?
CHAR,VARCHAR2,NUMBER,DATE,RAW,LONG and LONG RAW
Q)What is the maximum number of triggers, can apply to a single table?
Insert/Update/Delete :- 3
Before/After:- 2
Row Level/Statement Level:-2
Hence 3*2*2
Q)What is an integrity constraint?
Integrity constraint restricts values of a column so that only meaningful values as per the business logic of the table be entered into the column.
Integrity constraints are of the following types:
Not null
Unique
Primary Key
Foreign Key (referential integrity)
Check -- custom constraints e.g. check sal > 0, check joiningdate > date()
constraints; which enforces rules on a table columns which stops to enter unnecessary data
while inserting data or updating the data there are 5 types of constraints;
1) not null;This constraint doesn't allow null values.
2)Unique; The constraint associated with unique column keeps allow uniquely.
3)Primary key(primary+notnull) This constraint doesn't allow repeated values & null values.
4)check;This constraint checks the condition weather satisfied or not.
5)Foreign key;This constraint is used as reference key between the columns of two tables
Q)How do I replace a comma (,) with a blank in a select statement?
select empno||' '||ename from emp;
Display the number value in Words
select sal, (to_char(to_date(sal,'j'), 'jsp'))
from emp;
Q)If a view on a single base table is manipulated will the changes be reflected on the base table
an Insert,Update,delete can be done through views is
1. inserts/updates/deletes done only in one base table at a time
2. primary key columns should be part of view
3. columns which are going to be updated must be part of vew def.
4. it shouldn't voielet the contraints
Q)How to access the current value and next value from a sequence
I would like to give you a small example to use the sequence.currval and sequence.nextval
create sequence seq_name
start with 1
minvalue 1
maxvalue 999
increment by 1
nocycle
insert into table_name (sno,name) values (seqname.nextval,'abc');
select seqname.currval from dual
Q)If unique key constraint on DATE column is created, will it validate the rows that are inserted with SYSDATE
If there is a unique key defined on a column which has date as the data type then one can insert the same date more than once.The date is always stored in the format dd-mon-yyyy hh:mi:ss.The reason why it will accept the same date again is that the time taken for you to enter the date for the first time and entering the date again for the second time there will be a gap of atleast one secon which makes that date value unique. ONE CAN INSERT THE SAME DATE VALUE IN A UNIQUE COLUMN MORE THAN ONCE.
Q)Where the integrity constraints are stored in data dictionary
The integrity constraints are stored in USER_CONSTRAINTS.
Q)Is it possible to use LONG columns in WHERE clause or ORDER BY
No
Q)Difference between SQL and SQL* Plus
SQL*PLUS is a command line tool where as SQL and PL/SQL language interface and reporting tool. Its a command line tool that allows user to type SQL commands to be executed directly against an Oracle database. SQL is a language used to query the relational database(DML,DCL,DDL). SQL*PLUS commands are used to format query result, Set options, Edit SQL commands and PL/SQL.
Q)Difference between procedure and function.
Functions are named PL/SQL blocks that return a value and can be called with arguments procedure a named block that can be called with parameter. A procedure all is a PL/SQL statement by itself, while a Function call is called as part of an expression.
Q)What is an UTL_FILE.What are different procedures and functions associated with it?
UTL_FILE is a package that adds the ability to read and write to operating system files. Procedures associated with it are FCLOSE, FCLOSE_ALL and 5 procedures to
output data to a file PUT, PUT_LINE, NEW_LINE, PUTF, FFLUSH.PUT, FFLUSH.PUT_LINE,FFLUSH.NEW_LINE. Functions associated with it are FOPEN, ISOPEN
Cursor Oracle uses work area to execute SQL statements and store processing information PL/SQL construct called a cursor lets you name a work area and access its stored information A cursor is a mechanism used to fetch more than one row in a Pl/SQl block.
Cursor is a variable.
it is similar to a 2D array .
used for processing multiple rows.
used for storing data temporarily.
Q)Which is more faster - IN or EXISTS?
EXISTS is more faster than IN because EXISTS returns a Boolean value whereas IN returns a value.
Q)What are different Oracle database objects?
TABLES
VIEWS,
INDEXES,
SYNONYMS,
SEQUENCES,
TIGGERS,
PACKAGES,
PROCEDURE,
FUNCTION,
Q)Can a primary key contain more than one columns?
primary key not allow null values,unique key allow only one null value.In table only one primary is allowed,unique key as many as it can allow. In each table each column it can allow only one primary key the same table more than one primary key then it is called
composite primary key
Q)What are different modes of parameters used in functions and procedures?
-IN
-OUT
-INOUT
Q)Difference between NO DATA FOUND and %NOTFOUND
NO DATA FOUND is an exception raised only for the SELECT....INTO statements when the where clause of the query does not match any rows. When the where clause of the explicit cursor does not match any rows the %NOTFOUND attribute is set to TRUE instead.
________________________________________
NO_DATA_FOUND is pre defind exception of oracle.
when we can't get any data
from the table for any query.
In that case we will get error.
%NOT FOUND is one of the attribute
of explicit cursor.
when can't get any data from explicit cursor in that case
%NOT FOUND will returns true
otherwise it returns false.
Outer Join
OUTER join means fetched all the rows that is based on condition,as well as retrieve all rows that is not based on condition.
Q)What is the purpose of a cluster?
Oracle does not allow a user to specifically locate tables, since that is a part of the function of the RDBMS. However, for the purpose of increasing performance, oracle allows a developer to create a CLUSTER. A CLUSTER provides a means for storing data from different tables together for faster retrieval than if the table placement were left to the RDBMS.
it's a temperory and logical memory allocatiopn where we can put several tables.
Q)What is difference between SUBSTR and INSTR?
SUBSTR:-
returns a specified portion of a string
eg:- SUBSTR('GANGAA',5)
->output GANGA
INSTR:-
provides character position
found in a string. eg INSTR('KHAIRATABAD','A',3)
->output 8 (3RD occurrence of 'A')
SUBSTR returns a specified portion of a string eg SUBSTR('BCDEF',4) output BCDE
INSTR provides character position in which a pattern is found in a string. eg INSTR('ABC-DC-F','-',2) output 7 (2nd occurence of '-')
Q)What is difference between UNIQUE and PRIMARY KEY constraints?
A table can have only one PRIMARY KEY whereas there can be any number of UNIQUE keys. The columns that compose PK are automatically define NOT NULL, whereas a column that compose a UNIQUE is not automatically defined to be mandatory must also specify the column is NOT NULL.
Q)There is a '%' sign in one field of a column. What will be the query to find it ?
'\' Should be used before '%'.
You need to use INSTR function in the WHERE clause as follows :
select ename from emp
where instr(ename,'%',1,1) > 1
Q)What is OCI. What are its uses?
Oracle Call Interface is a method of accesing database from a 3GL program. Uses--No precompiler is required,PL/SQL blocks are executed like other DML statements. The OCI library provides
--functions to parse SQL statemets
--bind input variables
--bind output variables
--execute statements
--fetch the results
Q)Can a function take OUT parameters. If not why?
No. A function has to return a value,an OUT parameter cannot return a value.
Q)How you open and close a cursor variable.Why it is required?
OPEN cursor variable FOR SELECT...Statement
CLOSE cursor variable In order to associate a cursor variable with a particular SELECT statement OPEN syntax is used. In order to free the resources used for the query CLOSE statement is used.
Q)What a SELECT FOR UPDATE cursor represent.
SELECT......FROM......FOR......UPDATE[OF column-reference][NOWAIT] The processing done in a fetch loop modifies the rows that have been retrieved by the
cursor. A convenient way of modifying the rows is done by a method with two parts: the FOR UPDATE clause in the cursor declaration, WHERE CURRENT OF CLAUSE
in an UPDATE or declaration statement.
Q)Difference between foreign key and primary key?
1)Primary key is unique key but foriegn key always refers to
primary key.
2)Primary keys enforce entity integrity by uniquely
identifying entity instances. Foreign keys enforce.
3)Primary key is unique
primary key is not NULL and
foreign key is NULL
foreign key reference as Primary key in another table.
4) primary key is used to identify a row and it doesnot allow
null values. It avoids duplication of rows.
whereas foreign key refers to a column or combination of
columns included in the defenition of referential integrity.
referential integrity by completing an association between
two entities.
Q)What should be the return type for a cursor variable.Can we use a scalar data type as return type?
The return type for a cursor must be a record type.It can be declared explicitly as a user-defined or %ROWTYPE can be used. eg TYPE t_studentsref IS REF CURSOR
RETURN students%ROWTYPE
Q)Can you use a commit statement within a database trigger?
no,we can't use commit statement in the trigger,we are using trigger for checking purpose.
Q)What is difference between a formal and an actual parameter?
The variables declared in the procedure and which are passed, as arguments are called actual, the parameters in the procedure declaration. Actual parameters contain the values that are passed to a procedure and receive results. Formal parameters are the placeholders for the values of actual parameters
Q)What is a cursor for loop?
Cursor For Loop is a loop where oracle implicitly declares a loop variable, the loop index that of the same record type as the cursor's record.
________________________________________
When we use for loops,there is no need to declare explicit cursor.
Only case of for loop,cursor is implicitly open and after fetching the data cursor is implicitly closed.
Example given below
SET SERVEROUTPUT ON
BEGIN
FOR emp_record IN (SELECT last_name,department_id FROM employees) LOOP
IF emp_record.department_id=80 THEN
DBMS_OUTPUT.PUT_LINE('employees'||'emp_record.last_name'||'works for sales department');
END IF;
END LOOP;--implicit cursor closed
END;
Q)Difference between an implicit & an explicit cursor.
PL/SQL declares a cursor implicitly for all SQL data manipulation statements, including quries that return only one row. However,queries that return more than one row you must declare an explicit cursor or use a cursor FOR loop. Explicit cursor is a cursor in which the cursor name is explicitly assigned to a SELECT statement via the CURSOR...IS statement. An implicit cursor is used for all SQL
statements Declare, Open, Fetch, Close. An explicit cursors are used to process multirow SELECT statements An implicit cursor is used to process INSERT, UPDATE, DELETE and single row SELECT. .INTO statements.
________________________________________
The implicit cursor is used to process INSERT, UPDATE,
DELETE, and SELECT INTO statements. During the processing of
an implicit cursor,Oracle automatically performs the OPEN,
FETCH, and CLOSE operations.
Where as in explicit cursors,the process of its working is
done in 4 steps namely DECLARE a cursor,OPEN a cursor,
FETCH from cursor and CLOSE a
cursor.
IMPLICT CURSOR:- Automatically porvide by oracle which
perform DML statements. queries returns only one row.
EXPLICT CURSOR:- Defined by user. queries returns more
than rows.
Explicit Cursor:-We are not able to Handle NO_DATA_FOUND
Exception.
Implicit Cursor:-We are able to Handle NO_DATA_FOUND
Exception.
Q)There is a string 120000 12 0 .125 , how you will find the position of the decimal place?
INSTR('120000 12 0 .125',1,'.')
output 13
Q)Which datatype is used for storing graphics and images?
LONG RAW data type is used for storing BLOB's (binary large objects).
Q)When do you use WHERE clause and when do you use HAVING clause?
HAVING clause is used when you want to specify a condition for a group function and it is written after GROUP BY clause. The WHERE clause is used when you want to specify a condition for columns, single row functions except group functions and it is written before GROUP BY clause if it is used.
1.where clause is used to restrict the row ,basically where clause operate on single row function
2.Having clause is used to restrict the groups that means which groups to be displayed.
where is used to restrict the rows
having is used to restrict the group
Q)What is difference between Rename and Alias?
Rename is a permanent name given to a table or column whereas Alias is a temporary name given to a table or column which do not exist once the SQL statement is executed.
Rename;This is a DDL command which is used to the change the name of an object whereas alias is the alternate name given to the table while selecting this alias is used generally to join a same table.
EMPNO ENAME JOB MGR HIREDATE SAL COMM DEPTNO
7369 SMITH CLERK 7902 17-DEC-80 800 20
7499 ALLEN SALESMAN7698 20-FEB-81 1600 300 30
7521 WARD SALESMAN7698 22-FEB-81 1250 500 30
7566 JONES MANAGER 7839 02-APR-81 2975 20
7654 MARTIN SALESMAN7698 28-SEP-81 1250 1400 30
7698 BLAKE MANAGER 7839 01-MAY-81 2850 30
7782 CLARK MANAGER 7839 09-JUN-81 2450 10
7788 SCOTT ANALYST 7566 19-APR-87 3000 20
7839 KING PRESIDENT 17-NOV-81 5000 10
7844 TURNER SALESMAN7698 08-SEP-81 1500 0 30
7876 ADAMS CLERK 7788 23-MAY-87 1100 20
7900 JAMES CLERK 7698 03-DEC-81 950 30
7902 FORD ANALYST 7566 03-DEC-81 3000 20
7934 MILLER CLERK 7782 23-JAN-82 1300 10
Dept :
DEPTNO DNAME LOC
10 ACCOUNTING NEW YORK
20 RESEARCH DALLAS
30 SALES CHICAGO
40 OPERATIONS BOSTON
SALGrade:
GRADE LOSAL HISAL
1 700 1200
2 1201 1400
3 1401 2000
4 2001 3000
5 3001 9999
Q)How do I eliminate the duplicate rows ?
delete from table_name where rowid not in (select max(rowid) from table group by duplicate_values_field_name);
select max(rowid) from scott.emp group by job
Looks like 5 rows will remain after the deletions
MAX(ROWID)
AAAMgHAAEAAAAAeAAG
AAAMgHAAEAAAAAeAAM
AAAMgHAAEAAAAAeAAI
AAAMgHAAEAAAAAeAAJ
AAAMgHAAEAAAAAeAAN
Then get rid of the remaining duplicate rows.
Delete from emp where rowed not in(select max(rowid) from scott.emp group by job )
Q)How to identify the duplicate rows ?
select job, count(*) from scott.emp group by job having count(*)>1
JOB COUNT(*)
MANAGER 3
ANALYST 2
SALESMAN4
CLERK 4
Q)How do I display row number with records?
To achive this use rownum pseudocolumn with query, like SQL> SQL> select rownum, ename from emp;
Output:
1 Scott
2 Millor
3 Jiyo
4 Smith
Q)if commission is null then the text “Not Applicable” want to display, instead of blank space. How do I write the query?
select nvl(to_char(comm),'NA') from scott.emp;
NVL(TO_CHAR(COMM),'NA')
NA
300
500
NA
1400
NA
NA
NA
NA
0
NA
NA
NA
NA
Q)Display the records between two range
select rownum, empno, ename from scott.emp where rowid in
(select rowid from scott.emp where rownum <=14
minus
select rowid from scott.emp where rownum<7);
ROWNUM EMPNO ENAME
1 7782 CLARK
2 7788 SCOTT
3 7839 KING
4 7844 TURNER
5 7876 ADAMS
6 7900 JAMES
7 7902 FORD
8 7934 MILLER
Q)Oracle cursor : Implicit & Explicit cursors
Oracle uses work areas called private SQL areas to create SQL statements.
PL/SQL construct to identify each and every work are used, is called as Cursor.
For SQL queries returning a single row, PL/SQL declares all implicit cursors.
For queries that returning more than one row, the cursor needs to be explicitly declared.
Q)Explicit Cursor attributes
There are four cursor attributes used in Oracle
cursor_name%Found, cursor_name%NOTFOUND, cursor_name%ROWCOUNT, cursor_name%ISOPEN
Q)Implicit Cursor attributes
Same as explicit cursor but prefixed by the word SQL
SQL%Found, SQL%NOTFOUND, SQL%ROWCOUNT, SQL%ISOPEN
Q)What is the maximum number of triggers, can apply to a single table?
12 triggers
Q)What are PL/SQL Cursor Exceptions?
Cursor_Already_Open, Invalid_Cursor
Q)Any three PL/SQL Exceptions?
Too_many_rows, No_Data_Found, Value_Error, Zero_Error, Others
Q)Display Odd/ Even number of records
Odd number of records:
select * from emp where (rowid,1) in (select rowid, mod(rownum,2) from emp);
Even number of records:
select * from emp where (rowid,0) in (select rowid, mod(rownum,2) from emp)
Q)Find out nth highest salary from emp table
SELECT DISTINCT (a.sal) FROM EMP A WHERE &N = (SELECT COUNT (DISTINCT (b.sal)) FROM EMP B WHERE a.sal<=b.sal);
For explanation let a table emp with sal column like below
sal
1200
1300
1500
1200
1250
1700
1250
2000
See DISTINCT word is there in the query
So, you have to find the distinct sal first.
sal
1200
1300
1500
1250
1700
2000
Now see the condition a.sal<= b.sal
This condition compares a.sal and b.sal. The COUNT counts
how many times the a.sal is less than or equal to b.sal,
and gives this value as the output of sub query.
(N.B. comparing to same value means EQUAL SO count is 1).
This count is the value of N.
So after comparision the value of N for different salaries
are like
sal N
1200 6
1300 4
1500 3
1250 5
1700 2
2000 1
Now on querying when you give a value of N the
corresponding value of salary is selected.
Like if you are giving N=2 , then 1700 is displayed.
Q)What is correlated sub-query ?
Correlated sub query is a sub query which has reference to the main query
Q)What is a Non- corelated subquery
Non corelated subquery, where the subquery has been executed once for the entire parent statement.
Q)What are the usage of SAVEPOINTS ?
SAVEPOINTS are used to subdivide a transaction into smaller parts. It enables rolling back part of a transaction. Maximum of five save points are allowed.
Q)What is meant by Scrollable cursor
A scrollable cursor, however, can move forward and backward, and can seek any desired record in the cursor. Such operations are common in applications that present results sets in scrolling windows. With a scrollable cursor, application developers do not need to create and manage their own buffer for the records.
Q)What are the more common pseudo-columns
SELECT rownum, rowid FROM sometable
pseudo column is a column that is not present in the database.
it is used for unique identification of rows like
ROWNUM,ROWID,LEVEL,NEXTVAL,CURVAL,NULL etc.
Q)Where the integrity constraints are stored in Data Dictionary ?
The integrity constraints are stored in USER_CONSTRAINTS.
Q)What is ON DELETE CASCADE ?
When ON DELETE CASCADE is specified ORACLE maintains referential integrity by automatically removing dependent foreign key values if a referenced primary or unique key value is removed.
Q)What is a join ? Explain the different types of joins ?
A join is used to combine rows from multiple tables. A join is performed whenever two or more tables is listed in the FROM clause of an SQL statement.
The types of joins that you learn are
EQUIJOINS
NATURAL JOINS
NON-EQUIJOINS
OUTER JOINS
SELF JOINS
Equijoin : Perhaps the most used and important of the joins is the EQUIJOIN, also referred to as an INNER JOIN. The EQUIJOIN joins two tables with a common column in which each is usually the primary key.
SELECT EMPLOYEE_TBL.EMP_ID,
EMPLOYEE_PAY_TBL.DATE_HIRE
FROM EMPLOYEE_TBL,
EMPLOYEE_PAY_TBL
WHERE EMPLOYEE_TBL.EMP_ID = EMPLOYEE_PAY_TBL.EMP_ID;
Natural Join :
A natural join is a join statement that compares the common columns of both tables with each other. One should check whether common columns exist in both tables before doing a natural join.
Natural joins may cause problems if columns are added or renamed. Also, no more than two tables can be joined using this method. So, it is best to avoid natural joins as far as possible.
Examples
This is the same as an equi join on (emp.deptno = dept.deptno):
SELECT dname, ename FROM dept NATURAL JOIN emp
Same query on the HR sample schema:
SELECT department_name, first_name||' '||last_name
FROM departments NATURAL JOIN employees;
Nonequi join:
An nonequi (or theta) join is a join statement that uses an unequal operation (i.e: <>, >, <, !=, BETWEEN, etc.) to match rows from different tables. The converse of an nonequi join is a equi join operation.
Using Oracle join syntax:
SELECT e.ename, e.sal, s.grade
FROM emp e, salgrade s
WHERE e.sal BETWEEN s.losal and s.hisal;
Using ANSI join syntax:
SELECT e.ename, e.sal, s.grade
FROM emp e INNER JOIN salgrade s
ON e.sal BETWEEN s.losal and s.hisal;
Self join:
A self join is a join in which a table is joined with itself. For example, when you require details about an employee and his manager (also an employee).
Oracle join syntax:
SELECT e1.ename||' works for '||e2.ename "Employees and their Managers"
FROM emp e1, emp e2
WHERE e1.mgr = e2.empno;
ANSI join syntax (Oracle 9i and above):
SELECT e1.ename||' works for '||e2.ename "Employees and their Managers"
FROM emp e1
JOIN emp e2
ON (e1.mgr = e2.empno);
Outer Joins :
An outer join extends the result of a simple join. An outer join returns all rows that satisfy the join condition and also returns some or all of those rows from one table for which no rows from the other satisfy the join condition.
To write a query that performs an outer join of tables A and B and returns all rows from A (a left outer join), use the ANSI LEFT [OUTER] JOIN syntax, or apply the outer join operator (+) to all columns of B in the join condition. For all rows in A that have no matching rows in B, Oracle returns null for any select list expressions containing columns of B.
To write a query that performs an outer join of tables A and B and returns all rows from B (a right outer join), use the ANSI RIGHT [OUTER] syntax, or apply the outer join operator (+) to all columns of A in the join condition. For all rows in B that have no matching rows in A, Oracle returns null for any select list expressions containing columns of A.
To write a query that performs an outer join and and returns all rows from A and B, extended with nulls if they do not satisfy the join condition (a full outer join), use the ANSI FULL [OUTER] JOIN syntax.
For example the following query returns all the employees and department names and even those department names where no employee is working.
select e.empno,e.ename,e.sal,e.deptno,d.dname,d.city from emp e, dept d where e.deptno(+)=d.deptno;
That is specify the (+) sign to the column which is lacking values.
Cartesian Products :
If two tables in a join query have no join condition, Oracle returns their Cartesian product. Oracle combines each row of one table with each row of the other. A Cartesian product always generates many rows and is rarely useful. For example, the Cartesian product of two tables, each with 100 rows, has 10,000 rows. Always include a join condition unless you specifically need a Cartesian product
Q)What is the Subquery ?
Sub query is a query whose return values are used in filtering conditions of the main query. Subqueries will be executed once for the entire parent statement.
Q)Can a view be updated/inserted/deleted? If Yes under what conditions ?
A View can be updated/deleted/inserted if it has only one base table if the view is based on columns from one or more tables then insert, update and delete is not possible.
Q)How many LONG columns are allowed in a table ? Is it possible to use LONG columns in WHERE clause or ORDER BY ?
Only one LONG columns is allowed. It is not possible to use LONG column in WHERE or ORDER BY clause.
Q)What is Referential Integrity ?
Maintaining data integrity through a set of rules that restrict the values of one or more columns of the tables based on the values of primary key or unique key of the referenced table.
Q)What is ROWID ?
ROWID is a pseudo column attached to each row of a table. It is 18 character long, blockno, rownumber are the components of ROWID.
Q)Drop the column
There are 2 ways of doing it :
alter table"table name" drop column "column name";
This drops the column immediately. However if there is huge data and you would like to postpone the task of dropping the columns you can make the columns unused and drop the unused columns during the weekend or less peak activity time.
Q)UNION,INTERSECT,MINUS,UNION ALL ?
Then, a few values are inserted:
alter session set nls_date_format='dd.mm.yyyy';
insert into table_1 values ( 3, 'hello' , to_date('28.08.1970'));
insert into table_1 values ( 42, 'galaxy', to_date('01.01.2001'));
insert into table_1 values (100, 'bye' , to_date('09.02.2004'));
insert into table_2 values ( 3, 'bye' , to_date('28.08.1970'));
insert into table_2 values ( 42, 'galaxy', to_date('01.01.2001'));
insert into table_2 values ( 60, 'bye' , to_date('09.02.2004'));
insert into table_2 values ( 3, 'hello' , to_date('05.05.2002'));
union all:
union all selects all rows from all select statements:
select col_1, col_2, col_3 from table_1 union all
select col_1, col_2, col_3 from table_2;
As can be seen, all records of both tables are returned:
COL_1 COL_2 COL_3
---------- ---------- ----------
3 hello 28.08.1970
42 galaxy 01.01.2001
100 bye 09.02.2004
3 bye 28.08.1970
42 galaxy 01.01.2001
60 bye 09.02.2004
3 hello 05.05.2002
union :
union all is very similar to union, however, it dismisses duplicate rows found across different select statements:
select col_1, col_2, col_3 from table_1 union
select col_1, col_2, col_3 from table_2;
The galaxy record is a duplicate. Hence, it is returned only once:
COL_1 COL_2 COL_3
---------- ---------- ----------
3 bye 28.08.1970
3 hello 28.08.1970
3 hello 05.05.2002
42 galaxy 01.01.2001
60 bye 09.02.2004
100 bye 09.02.2004
intersect:
intersect only returns the rows that are found in all select statements:
select col_1, col_2, col_3 from table_1 intersect
select col_1, col_2, col_3 from table_2;
Only the galaxy record is returned. It's the only record that is stored in both tables:
COL_1 COL_2 COL_3
---------- ---------- ----------
42 galaxy 01.01.2001
minus:
minus returns all rows from the first select statements except those who are duplicated in a following select statement:
select col_1, col_2, col_3 from table_1 minus
select col_1, col_2, col_3 from table_2;
As the galaxy record is found in both tables, it is removed from the first table's record set:
COL_1 COL_2 COL_3
---------- ---------- ----------
3 hello 28.08.1970
100 bye 09.02.2004
The following query is valid:
SELECT 3 FROM DUAL
INTERSECT
SELECT 3f FROM DUAL;
This is implicitly converted to the following compound query:
SELECT TO_BINARY_FLOAT(3) FROM DUAL
INTERSECT
SELECT 3f FROM DUAL;
The following query returns an error:
SELECT '3' FROM DUAL
INTERSECT
SELECT 3f FROM DUAL;
Q)What is CYCLE/NO CYCLE in a Sequence ?
CYCLE specifies that the sequence continues to generate values after reaching either maximum or minimum value. After pan ascending sequence reaches its maximum value, it generates its minimum value. After a descending sequence reaches its minimum, it generates its maximum.NO CYCLE specifies that the sequence cannot generate more values after reaching its maximum or minimum value.
Q)How will you a activate/deactivate integrity constraints ?
The integrity constraints can be enabled or disabled by ALTER TABLE ENABLE constraint/DISABLE constraint.
Q)What is difference between CHAR and VARCHAR2 ? What is the maximum SIZE allowed for each type ?
CHAR pads blank spaces to the maximum length. VARCHAR2 does not pad blank spaces. For CHAR it is 255 and 2000 for VARCHAR2.
Q)Database link
Database Link is a named path through which a remote database can be accessed.
Q)What are the data types allowed in a table ?
CHAR,VARCHAR2,NUMBER,DATE,RAW,LONG and LONG RAW
Q)What is the maximum number of triggers, can apply to a single table?
Insert/Update/Delete :- 3
Before/After:- 2
Row Level/Statement Level:-2
Hence 3*2*2
Q)What is an integrity constraint?
Integrity constraint restricts values of a column so that only meaningful values as per the business logic of the table be entered into the column.
Integrity constraints are of the following types:
Not null
Unique
Primary Key
Foreign Key (referential integrity)
Check -- custom constraints e.g. check sal > 0, check joiningdate > date()
constraints; which enforces rules on a table columns which stops to enter unnecessary data
while inserting data or updating the data there are 5 types of constraints;
1) not null;This constraint doesn't allow null values.
2)Unique; The constraint associated with unique column keeps allow uniquely.
3)Primary key(primary+notnull) This constraint doesn't allow repeated values & null values.
4)check;This constraint checks the condition weather satisfied or not.
5)Foreign key;This constraint is used as reference key between the columns of two tables
Q)How do I replace a comma (,) with a blank in a select statement?
select empno||' '||ename from emp;
Display the number value in Words
select sal, (to_char(to_date(sal,'j'), 'jsp'))
from emp;
Q)If a view on a single base table is manipulated will the changes be reflected on the base table
an Insert,Update,delete can be done through views is
1. inserts/updates/deletes done only in one base table at a time
2. primary key columns should be part of view
3. columns which are going to be updated must be part of vew def.
4. it shouldn't voielet the contraints
Q)How to access the current value and next value from a sequence
I would like to give you a small example to use the sequence.currval and sequence.nextval
create sequence seq_name
start with 1
minvalue 1
maxvalue 999
increment by 1
nocycle
insert into table_name (sno,name) values (seqname.nextval,'abc');
select seqname.currval from dual
Q)If unique key constraint on DATE column is created, will it validate the rows that are inserted with SYSDATE
If there is a unique key defined on a column which has date as the data type then one can insert the same date more than once.The date is always stored in the format dd-mon-yyyy hh:mi:ss.The reason why it will accept the same date again is that the time taken for you to enter the date for the first time and entering the date again for the second time there will be a gap of atleast one secon which makes that date value unique. ONE CAN INSERT THE SAME DATE VALUE IN A UNIQUE COLUMN MORE THAN ONCE.
Q)Where the integrity constraints are stored in data dictionary
The integrity constraints are stored in USER_CONSTRAINTS.
Q)Is it possible to use LONG columns in WHERE clause or ORDER BY
No
Q)Difference between SQL and SQL* Plus
SQL*PLUS is a command line tool where as SQL and PL/SQL language interface and reporting tool. Its a command line tool that allows user to type SQL commands to be executed directly against an Oracle database. SQL is a language used to query the relational database(DML,DCL,DDL). SQL*PLUS commands are used to format query result, Set options, Edit SQL commands and PL/SQL.
Q)Difference between procedure and function.
Functions are named PL/SQL blocks that return a value and can be called with arguments procedure a named block that can be called with parameter. A procedure all is a PL/SQL statement by itself, while a Function call is called as part of an expression.
Q)What is an UTL_FILE.What are different procedures and functions associated with it?
UTL_FILE is a package that adds the ability to read and write to operating system files. Procedures associated with it are FCLOSE, FCLOSE_ALL and 5 procedures to
output data to a file PUT, PUT_LINE, NEW_LINE, PUTF, FFLUSH.PUT, FFLUSH.PUT_LINE,FFLUSH.NEW_LINE. Functions associated with it are FOPEN, ISOPEN
Cursor Oracle uses work area to execute SQL statements and store processing information PL/SQL construct called a cursor lets you name a work area and access its stored information A cursor is a mechanism used to fetch more than one row in a Pl/SQl block.
Cursor is a variable.
it is similar to a 2D array .
used for processing multiple rows.
used for storing data temporarily.
Q)Which is more faster - IN or EXISTS?
EXISTS is more faster than IN because EXISTS returns a Boolean value whereas IN returns a value.
Q)What are different Oracle database objects?
TABLES
VIEWS,
INDEXES,
SYNONYMS,
SEQUENCES,
TIGGERS,
PACKAGES,
PROCEDURE,
FUNCTION,
Q)Can a primary key contain more than one columns?
primary key not allow null values,unique key allow only one null value.In table only one primary is allowed,unique key as many as it can allow. In each table each column it can allow only one primary key the same table more than one primary key then it is called
composite primary key
Q)What are different modes of parameters used in functions and procedures?
-IN
-OUT
-INOUT
Q)Difference between NO DATA FOUND and %NOTFOUND
NO DATA FOUND is an exception raised only for the SELECT....INTO statements when the where clause of the query does not match any rows. When the where clause of the explicit cursor does not match any rows the %NOTFOUND attribute is set to TRUE instead.
________________________________________
NO_DATA_FOUND is pre defind exception of oracle.
when we can't get any data
from the table for any query.
In that case we will get error.
%NOT FOUND is one of the attribute
of explicit cursor.
when can't get any data from explicit cursor in that case
%NOT FOUND will returns true
otherwise it returns false.
Outer Join
OUTER join means fetched all the rows that is based on condition,as well as retrieve all rows that is not based on condition.
Q)What is the purpose of a cluster?
Oracle does not allow a user to specifically locate tables, since that is a part of the function of the RDBMS. However, for the purpose of increasing performance, oracle allows a developer to create a CLUSTER. A CLUSTER provides a means for storing data from different tables together for faster retrieval than if the table placement were left to the RDBMS.
it's a temperory and logical memory allocatiopn where we can put several tables.
Q)What is difference between SUBSTR and INSTR?
SUBSTR:-
returns a specified portion of a string
eg:- SUBSTR('GANGAA',5)
->output GANGA
INSTR:-
provides character position
found in a string. eg INSTR('KHAIRATABAD','A',3)
->output 8 (3RD occurrence of 'A')
SUBSTR returns a specified portion of a string eg SUBSTR('BCDEF',4) output BCDE
INSTR provides character position in which a pattern is found in a string. eg INSTR('ABC-DC-F','-',2) output 7 (2nd occurence of '-')
Q)What is difference between UNIQUE and PRIMARY KEY constraints?
A table can have only one PRIMARY KEY whereas there can be any number of UNIQUE keys. The columns that compose PK are automatically define NOT NULL, whereas a column that compose a UNIQUE is not automatically defined to be mandatory must also specify the column is NOT NULL.
Q)There is a '%' sign in one field of a column. What will be the query to find it ?
'\' Should be used before '%'.
You need to use INSTR function in the WHERE clause as follows :
select ename from emp
where instr(ename,'%',1,1) > 1
Q)What is OCI. What are its uses?
Oracle Call Interface is a method of accesing database from a 3GL program. Uses--No precompiler is required,PL/SQL blocks are executed like other DML statements. The OCI library provides
--functions to parse SQL statemets
--bind input variables
--bind output variables
--execute statements
--fetch the results
Q)Can a function take OUT parameters. If not why?
No. A function has to return a value,an OUT parameter cannot return a value.
Q)How you open and close a cursor variable.Why it is required?
OPEN cursor variable FOR SELECT...Statement
CLOSE cursor variable In order to associate a cursor variable with a particular SELECT statement OPEN syntax is used. In order to free the resources used for the query CLOSE statement is used.
Q)What a SELECT FOR UPDATE cursor represent.
SELECT......FROM......FOR......UPDATE[OF column-reference][NOWAIT] The processing done in a fetch loop modifies the rows that have been retrieved by the
cursor. A convenient way of modifying the rows is done by a method with two parts: the FOR UPDATE clause in the cursor declaration, WHERE CURRENT OF CLAUSE
in an UPDATE or declaration statement.
Q)Difference between foreign key and primary key?
1)Primary key is unique key but foriegn key always refers to
primary key.
2)Primary keys enforce entity integrity by uniquely
identifying entity instances. Foreign keys enforce.
3)Primary key is unique
primary key is not NULL and
foreign key is NULL
foreign key reference as Primary key in another table.
4) primary key is used to identify a row and it doesnot allow
null values. It avoids duplication of rows.
whereas foreign key refers to a column or combination of
columns included in the defenition of referential integrity.
referential integrity by completing an association between
two entities.
Q)What should be the return type for a cursor variable.Can we use a scalar data type as return type?
The return type for a cursor must be a record type.It can be declared explicitly as a user-defined or %ROWTYPE can be used. eg TYPE t_studentsref IS REF CURSOR
RETURN students%ROWTYPE
Q)Can you use a commit statement within a database trigger?
no,we can't use commit statement in the trigger,we are using trigger for checking purpose.
Q)What is difference between a formal and an actual parameter?
The variables declared in the procedure and which are passed, as arguments are called actual, the parameters in the procedure declaration. Actual parameters contain the values that are passed to a procedure and receive results. Formal parameters are the placeholders for the values of actual parameters
Q)What is a cursor for loop?
Cursor For Loop is a loop where oracle implicitly declares a loop variable, the loop index that of the same record type as the cursor's record.
________________________________________
When we use for loops,there is no need to declare explicit cursor.
Only case of for loop,cursor is implicitly open and after fetching the data cursor is implicitly closed.
Example given below
SET SERVEROUTPUT ON
BEGIN
FOR emp_record IN (SELECT last_name,department_id FROM employees) LOOP
IF emp_record.department_id=80 THEN
DBMS_OUTPUT.PUT_LINE('employees'||'emp_record.last_name'||'works for sales department');
END IF;
END LOOP;--implicit cursor closed
END;
Q)Difference between an implicit & an explicit cursor.
PL/SQL declares a cursor implicitly for all SQL data manipulation statements, including quries that return only one row. However,queries that return more than one row you must declare an explicit cursor or use a cursor FOR loop. Explicit cursor is a cursor in which the cursor name is explicitly assigned to a SELECT statement via the CURSOR...IS statement. An implicit cursor is used for all SQL
statements Declare, Open, Fetch, Close. An explicit cursors are used to process multirow SELECT statements An implicit cursor is used to process INSERT, UPDATE, DELETE and single row SELECT. .INTO statements.
________________________________________
The implicit cursor is used to process INSERT, UPDATE,
DELETE, and SELECT INTO statements. During the processing of
an implicit cursor,Oracle automatically performs the OPEN,
FETCH, and CLOSE operations.
Where as in explicit cursors,the process of its working is
done in 4 steps namely DECLARE a cursor,OPEN a cursor,
FETCH from cursor and CLOSE a
cursor.
IMPLICT CURSOR:- Automatically porvide by oracle which
perform DML statements. queries returns only one row.
EXPLICT CURSOR:- Defined by user. queries returns more
than rows.
Explicit Cursor:-We are not able to Handle NO_DATA_FOUND
Exception.
Implicit Cursor:-We are able to Handle NO_DATA_FOUND
Exception.
Q)There is a string 120000 12 0 .125 , how you will find the position of the decimal place?
INSTR('120000 12 0 .125',1,'.')
output 13
Q)Which datatype is used for storing graphics and images?
LONG RAW data type is used for storing BLOB's (binary large objects).
Q)When do you use WHERE clause and when do you use HAVING clause?
HAVING clause is used when you want to specify a condition for a group function and it is written after GROUP BY clause. The WHERE clause is used when you want to specify a condition for columns, single row functions except group functions and it is written before GROUP BY clause if it is used.
1.where clause is used to restrict the row ,basically where clause operate on single row function
2.Having clause is used to restrict the groups that means which groups to be displayed.
where is used to restrict the rows
having is used to restrict the group
Q)What is difference between Rename and Alias?
Rename is a permanent name given to a table or column whereas Alias is a temporary name given to a table or column which do not exist once the SQL statement is executed.
Rename;This is a DDL command which is used to the change the name of an object whereas alias is the alternate name given to the table while selecting this alias is used generally to join a same table.
Oracle SQL PL Sql Questions Answers
What are the indexes in Oracle ?
An index is a schema object that contains an entry for each value that appears in the indexed column(s) of the table or cluster and provides direct, fast access to rows. Oracle Database supports several types of index:
Normal indexes. (By default, Oracle Database creates B-tree indexes.)
Bitmap indexes, which store rowids associated with a key value as a bitmap
Partitioned indexes, which consist of partitions containing an entry for each value that appears in the indexed column(s) of the table
Function-based indexes, which are based on expressions. They enable you to construct queries that evaluate the value returned by an expression, which in turn may include built-in or user-defined functions.
Domain indexes, which are instances of an application-specific index of type indextype
Q)What is difference between IN and NOT IN ?
SQL: "IN" Function
________________________________________
The IN function helps reduce the need to use multiple OR conditions.
The syntax for the IN function is:
SELECT columns
FROM tables
WHERE column1 in (value1, value2, .... value_n);
This SQL statement will return the records where column1 is value1, value2..., or value_n. The IN function can be used in any valid SQL statement - select, insert, update, or delete.
Example #1
The following is an SQL statement that uses the IN function:
SELECT *
FROM suppliers
WHERE supplier_name in ( 'IBM', 'Hewlett Packard', 'Microsoft');
This would return all rows where the supplier_name is either IBM, Hewlett Packard, or Microsoft. Because the * is used in the select, all fields from the suppliers table would appear in the result set.
It is equivalent to the following statement:
SELECT *
FROM suppliers
WHERE supplier_name = 'IBM'
OR supplier_name = 'Hewlett Packard'
OR supplier_name = 'Microsoft';
As you can see, using the IN function makes the statement easier to read and more efficient.
Example #2
You can also use the IN function with numeric values.
SELECT *
FROM orders
WHERE order_id in (10000, 10001, 10003, 10005);
This SQL statement would return all orders where the order_id is either 10000, 10001, 10003, or 10005.
It is equivalent to the following statement:
SELECT *
FROM orders
WHERE order_id = 10000
OR order_id = 10001
OR order_id = 10003
OR order_id = 10005;
Example #3 using "NOT IN"
The IN function can also be combined with the NOT operator.
For example,
SELECT *
FROM suppliers
WHERE supplier_name not in ( 'IBM', 'Hewlett Packard', 'Microsoft');
This would return all rows where the supplier_name is neither IBM, Hewlett Packard, or Microsoft. Sometimes, it is more efficient to list the values that you do not want, as opposed to the values that you do want.
Q)Understanding how NULLS affect the IN,NOT IN , EXISTS and NOT EXISTS?
Answer 1:
On the surface, it may appear that the SQL clauses IN and EXISTS are interchangeable. However, they're quite different in how they handle NULL values and may give different results. The problem comes from the fact that, in an Oracle database, a NULL value means unknown, so any comparison or operation against a NULL value is also NULL, and any test that returns NULL is always ignored. For example, neither one of these queries return any rows:
select 'true' from dual where 1 = null; = No Result
select 'true' from dual where 1 != null; = No Result
The value 1 is neither equal nor not equal to NULL. Only IS NULL would return true on a NULL value and return a row.
select 'true' from dual where 1 is null; ; = No Result
select 'true' from dual where null is null; Return ‘true’
When you use IN, you're telling SQL to take a value and compare it against every value or set of values in a list using =. If any NULL values exist, a row will not be returned--even if both values are NULL.
select 'true' from dual where null in (null); = No Result
select 'true' from dual where (null,null) in ((null,null)); = No Result
select 'true' from dual where (1,null) in ((1,null)); = No Result
An IN is functionally equivalent to the = ANY clause:
select 'true' from dual where null = ANY (null); ); = No Result
select 'true' from dual where (null,null) = ANY ((null,null)); ); = No Result
select 'true' from dual where (1,null) = ANY ((1,null)); ); = No Result
When you use an equivalent form of EXISTS, SQL counts rows and ignores the value(s) in the subquery--even if you return NULL.
select 'true' from dual where exists (select null from dual); Return True
select 'true' from dual where exists (select 0 from dual where null is null); Return True
The IN and EXISTS are logically the same. The IN clause compares values returned by the subquery and filters out rows in the outer query; the EXISTS clause compares values and filters out rows inside the subquery. In the case of NULL values, the resulting set of rows is the same.
select ename from emp where empno in (select mgr from emp); Return Manager name
select ename from emp e where exists (select 0 from emp where mgr = e.empno); Return
Manager name
But problems arise when the logic is reversed to use NOT IN and NOT EXISTS, which return different sets of rows (the first query returns 0 rows; the second returns the intended data--they aren't the same query):
select ename from emp where empno not in (select mgr from emp); No rows returned because subquery return NULL
select ename from emp e where not exists (select 0 from emp where mgr =
e.empno);
The NOT IN clause is virtually equivalent to comparing each value with = and failing if any test is FALSE or NULL. For example:
select 'true' from dual where 1 not in (null,2);
select 'true' from dual where 1 != null and 1 != 2;
select 'true' from dual where (1,2) not in ((2,3),(2,null));
select 'true' from dual where (1,null) not in ((1,2),(2,3));
These queries don't return any rows. The second is more obvious, 1 != NULL is NULL, so the whole WHERE condition is false for that row. While these would work:
select 'true' from dual where 1 not in (2,3);
select 'true' from dual where 1 != 2 and 1 != 3;
You can still use the NOT IN query from before, as long as you prevent NULL from being returned in the results (again, these both work, but I'm assuming empno is not null, which is a good assumption in this case):
select ename from emp where empno not in (select mgr from emp where mgr is not
null);
select ename from emp where empno not in (select nvl(mgr,0) from emp);
By understanding the difference between IN, EXISTS, NOT IN, and NOT EXISTS, you can avoid a very common problem when NULLs appear in the data of a subquery.
Answer2 :
Resulting in:
EMP_NBR EMP_NAME MGR_NBR
1 DON 5
2 HARI 5
3 RAMESH 5
4 JOE 5
5 DENNIS NULL
6 NIMISH 5
7 JESSIE 5
8 KEN 5
9 AMBER 5
10 JIM 5
Now, the aim is to find all those employees who are not managers. Let’s see how we can achieve that by using the “NOT IN” vs the “NOT EXISTS” clause.
NOT IN
SQL> select count(*) from emp_master where emp_nbr not in ( select mgr_nbr from emp_master );
COUNT(*)
———-
0
This means that everyone is a manager…hmmm, I wonder whether anything ever gets done in that case
NOT EXISTS
SQL> select count(*) from emp_master T1 where not exists ( select 1 from emp_master T2 where t2.mgr_nbr = t1.emp_nbr );
COUNT(*)
———-
9
Now there are 9 people who are not managers. So, you can clearly see the difference that NULL values make and since NULL != NULL in SQL, the NOT IN clause does not return any records back. (in MS SQL Server, depending upon the ANSI NULLS setting, the behavior can be altered but this post only talks about the behavior that is same in Oracle, DB2 LUW and MS SQL Server).
Performance implications:
When using “NOT IN”, the query performs nested full table scans, whereas for “NOT EXISTS”, query can use an index within the sub-query.
Another Optional Method
Another way of doing this is to use an outer join and check for NULL values in the other table:
SELECT COUNT(*)
FROM EMP_MASTER T1
LEFT OUTER JOIN EMP_MASTER T2
ON T1.EMP_NBR = T2.MGR_NBR
WHERE T2.MGR_NBR IS NULL
/
Of course, there should be other selection criteria as well (possibly a range search criteria, an equality SARG (searchable argument) criteria etc.) to help improve the selectivity besides just the NOT EXISTS clause.
Answer 3:
What's the difference between NOT IN and NOT EXISTS in SQL? Which is better to use in SQL queries for the performance aspects?
> EXPERT RESPONSE
For the first question, let's assume we're talking about the difference in syntax between NOT IN and NOT EXISTS when both have subqueries.
Here are examples of the syntax:
select client_name
from clients
where client_id not in
( select client_id
from clientwarnings )
select client_name
from clients as x
where not exists
( select client_id
from clientwarnings
where client_id = x.client_id )
In the NOT IN example, the subquery identifies a set of client_ids, against which each client_id in the clients table can be compared. In the NOT EXISTS example, the subquery is correlated and can be thought of as doing a lookup for each client_id in the clients table.
Your second question asks which is preferred for performance reasons. My answer is: it's your call. Don't trust your instinct, because it might be wrong.
My guess would be that the NOT IN query has to build a list for the subquery, which is slow, but then it can do in-memory compares, which is fast, while the NOT EXISTS query can use matching indexes, which are faster than building a memory table ... As you can see, I am easily confused.
And what about the LEFT OUTER JOIN testing for IS NULL? A perfectly good alternate solution:
select c.client_name
from clients as c
left outer
join clientwarnings as cw
on c.client_id = cw.client_id
where cw.client_id is null
Answer 4 :
When coding a SQL statement with tables in master-detail relationships, it's common to have to decide whether to write the query using the WHERE EXISTS (. . .) clause or the WHERE value IN (. . .) clause. You may resist using WHERE EXISTS because it has the awkward syntax of returning a value, which you always ignore.
However, there's a difference when using rule-based optimization. You can determine the performance of a rule-based query by understanding which table is the driving table and how many rows each part returns.
When you write a query using the IN clause, you're telling the rule-based optimizer that you want the inner query to drive the outer query (think: IN = inside to outside). For example, to query the 14-row EMP table for the direct reports to the employee KING, you could write the following:
select ename from emp e
where mgr in (select empno from emp where ename = 'KING');
Here's the EXPLAIN PLAN for this query:
OBJECT OPERATION
---------- ----------------------------------------
SELECT STATEMENT()
NESTED LOOPS()
EMP TABLE ACCESS(FULL)
EMP TABLE ACCESS(BY INDEX ROWID)
PK_EMP INDEX(UNIQUE SCAN)
This query is virtually equivalent to this:
select e1.ename from emp e1,(select empno from emp where ename = 'KING') e2
where e1.mgr = e2.empno;
You can write the same query using EXISTS by moving the outer query column to a subquery condition, like this:
select ename from emp e
where exists (select 0 from emp where e.mgr = empno and ename = 'KING');
When you write EXISTS in a where clause, you're telling the optimizer that you want the outer query to be run first, using each value to fetch a value from the inner query (think: EXISTS = outside to inside).
The EXPLAIN PLAN result for the query is:
OBJECT OPERATION
---------- ----------------------------------------
SELECT STATEMENT()
FILTER()
EMP TABLE ACCESS(FULL)
EMP TABLE ACCESS(BY INDEX ROWID)
PK_EMP INDEX(UNIQUE SCAN)
This is virtually similar to the PL/SQL code:
set serveroutput on;
declare
l_count integer;
begin
for e in (select mgr,ename from emp) loop
select count(*) into l_count from emp
where e.mgr = empno and ename = 'KING';
if l_count != 0 then
dbms_output.put_line(e.ename);
end if;
end loop;
end;
To determine which clause offers better performance in rule-based optimization, consider how many rows the inner query will return in comparison to the outer query. In many cases, EXISTS is better because it requires you to specify a join condition, which can invoke an INDEX scan. However, IN is often better if the results of the subquery are very small. You usually want to run the query that returns the smaller set of results first.
Some people avoid the EXISTS clause because of the requirement to return a result from the query--even though the result is never used. Depending on personal style, people often use 'X,' 1, 0, or null. From looking at the EXPLAIN PLAN output, it appears that the optimizer throws out whatever value you enter and uses 0 all the time. Many developers get into the habit of always entering some constant value.
An index is a schema object that contains an entry for each value that appears in the indexed column(s) of the table or cluster and provides direct, fast access to rows. Oracle Database supports several types of index:
Normal indexes. (By default, Oracle Database creates B-tree indexes.)
Bitmap indexes, which store rowids associated with a key value as a bitmap
Partitioned indexes, which consist of partitions containing an entry for each value that appears in the indexed column(s) of the table
Function-based indexes, which are based on expressions. They enable you to construct queries that evaluate the value returned by an expression, which in turn may include built-in or user-defined functions.
Domain indexes, which are instances of an application-specific index of type indextype
Q)What is difference between IN and NOT IN ?
SQL: "IN" Function
________________________________________
The IN function helps reduce the need to use multiple OR conditions.
The syntax for the IN function is:
SELECT columns
FROM tables
WHERE column1 in (value1, value2, .... value_n);
This SQL statement will return the records where column1 is value1, value2..., or value_n. The IN function can be used in any valid SQL statement - select, insert, update, or delete.
Example #1
The following is an SQL statement that uses the IN function:
SELECT *
FROM suppliers
WHERE supplier_name in ( 'IBM', 'Hewlett Packard', 'Microsoft');
This would return all rows where the supplier_name is either IBM, Hewlett Packard, or Microsoft. Because the * is used in the select, all fields from the suppliers table would appear in the result set.
It is equivalent to the following statement:
SELECT *
FROM suppliers
WHERE supplier_name = 'IBM'
OR supplier_name = 'Hewlett Packard'
OR supplier_name = 'Microsoft';
As you can see, using the IN function makes the statement easier to read and more efficient.
Example #2
You can also use the IN function with numeric values.
SELECT *
FROM orders
WHERE order_id in (10000, 10001, 10003, 10005);
This SQL statement would return all orders where the order_id is either 10000, 10001, 10003, or 10005.
It is equivalent to the following statement:
SELECT *
FROM orders
WHERE order_id = 10000
OR order_id = 10001
OR order_id = 10003
OR order_id = 10005;
Example #3 using "NOT IN"
The IN function can also be combined with the NOT operator.
For example,
SELECT *
FROM suppliers
WHERE supplier_name not in ( 'IBM', 'Hewlett Packard', 'Microsoft');
This would return all rows where the supplier_name is neither IBM, Hewlett Packard, or Microsoft. Sometimes, it is more efficient to list the values that you do not want, as opposed to the values that you do want.
Q)Understanding how NULLS affect the IN,NOT IN , EXISTS and NOT EXISTS?
Answer 1:
On the surface, it may appear that the SQL clauses IN and EXISTS are interchangeable. However, they're quite different in how they handle NULL values and may give different results. The problem comes from the fact that, in an Oracle database, a NULL value means unknown, so any comparison or operation against a NULL value is also NULL, and any test that returns NULL is always ignored. For example, neither one of these queries return any rows:
select 'true' from dual where 1 = null; = No Result
select 'true' from dual where 1 != null; = No Result
The value 1 is neither equal nor not equal to NULL. Only IS NULL would return true on a NULL value and return a row.
select 'true' from dual where 1 is null; ; = No Result
select 'true' from dual where null is null; Return ‘true’
When you use IN, you're telling SQL to take a value and compare it against every value or set of values in a list using =. If any NULL values exist, a row will not be returned--even if both values are NULL.
select 'true' from dual where null in (null); = No Result
select 'true' from dual where (null,null) in ((null,null)); = No Result
select 'true' from dual where (1,null) in ((1,null)); = No Result
An IN is functionally equivalent to the = ANY clause:
select 'true' from dual where null = ANY (null); ); = No Result
select 'true' from dual where (null,null) = ANY ((null,null)); ); = No Result
select 'true' from dual where (1,null) = ANY ((1,null)); ); = No Result
When you use an equivalent form of EXISTS, SQL counts rows and ignores the value(s) in the subquery--even if you return NULL.
select 'true' from dual where exists (select null from dual); Return True
select 'true' from dual where exists (select 0 from dual where null is null); Return True
The IN and EXISTS are logically the same. The IN clause compares values returned by the subquery and filters out rows in the outer query; the EXISTS clause compares values and filters out rows inside the subquery. In the case of NULL values, the resulting set of rows is the same.
select ename from emp where empno in (select mgr from emp); Return Manager name
select ename from emp e where exists (select 0 from emp where mgr = e.empno); Return
Manager name
But problems arise when the logic is reversed to use NOT IN and NOT EXISTS, which return different sets of rows (the first query returns 0 rows; the second returns the intended data--they aren't the same query):
select ename from emp where empno not in (select mgr from emp); No rows returned because subquery return NULL
select ename from emp e where not exists (select 0 from emp where mgr =
e.empno);
The NOT IN clause is virtually equivalent to comparing each value with = and failing if any test is FALSE or NULL. For example:
select 'true' from dual where 1 not in (null,2);
select 'true' from dual where 1 != null and 1 != 2;
select 'true' from dual where (1,2) not in ((2,3),(2,null));
select 'true' from dual where (1,null) not in ((1,2),(2,3));
These queries don't return any rows. The second is more obvious, 1 != NULL is NULL, so the whole WHERE condition is false for that row. While these would work:
select 'true' from dual where 1 not in (2,3);
select 'true' from dual where 1 != 2 and 1 != 3;
You can still use the NOT IN query from before, as long as you prevent NULL from being returned in the results (again, these both work, but I'm assuming empno is not null, which is a good assumption in this case):
select ename from emp where empno not in (select mgr from emp where mgr is not
null);
select ename from emp where empno not in (select nvl(mgr,0) from emp);
By understanding the difference between IN, EXISTS, NOT IN, and NOT EXISTS, you can avoid a very common problem when NULLs appear in the data of a subquery.
Answer2 :
Resulting in:
EMP_NBR EMP_NAME MGR_NBR
1 DON 5
2 HARI 5
3 RAMESH 5
4 JOE 5
5 DENNIS NULL
6 NIMISH 5
7 JESSIE 5
8 KEN 5
9 AMBER 5
10 JIM 5
Now, the aim is to find all those employees who are not managers. Let’s see how we can achieve that by using the “NOT IN” vs the “NOT EXISTS” clause.
NOT IN
SQL> select count(*) from emp_master where emp_nbr not in ( select mgr_nbr from emp_master );
COUNT(*)
———-
0
This means that everyone is a manager…hmmm, I wonder whether anything ever gets done in that case
NOT EXISTS
SQL> select count(*) from emp_master T1 where not exists ( select 1 from emp_master T2 where t2.mgr_nbr = t1.emp_nbr );
COUNT(*)
———-
9
Now there are 9 people who are not managers. So, you can clearly see the difference that NULL values make and since NULL != NULL in SQL, the NOT IN clause does not return any records back. (in MS SQL Server, depending upon the ANSI NULLS setting, the behavior can be altered but this post only talks about the behavior that is same in Oracle, DB2 LUW and MS SQL Server).
Performance implications:
When using “NOT IN”, the query performs nested full table scans, whereas for “NOT EXISTS”, query can use an index within the sub-query.
Another Optional Method
Another way of doing this is to use an outer join and check for NULL values in the other table:
SELECT COUNT(*)
FROM EMP_MASTER T1
LEFT OUTER JOIN EMP_MASTER T2
ON T1.EMP_NBR = T2.MGR_NBR
WHERE T2.MGR_NBR IS NULL
/
Of course, there should be other selection criteria as well (possibly a range search criteria, an equality SARG (searchable argument) criteria etc.) to help improve the selectivity besides just the NOT EXISTS clause.
Answer 3:
What's the difference between NOT IN and NOT EXISTS in SQL? Which is better to use in SQL queries for the performance aspects?
> EXPERT RESPONSE
For the first question, let's assume we're talking about the difference in syntax between NOT IN and NOT EXISTS when both have subqueries.
Here are examples of the syntax:
select client_name
from clients
where client_id not in
( select client_id
from clientwarnings )
select client_name
from clients as x
where not exists
( select client_id
from clientwarnings
where client_id = x.client_id )
In the NOT IN example, the subquery identifies a set of client_ids, against which each client_id in the clients table can be compared. In the NOT EXISTS example, the subquery is correlated and can be thought of as doing a lookup for each client_id in the clients table.
Your second question asks which is preferred for performance reasons. My answer is: it's your call. Don't trust your instinct, because it might be wrong.
My guess would be that the NOT IN query has to build a list for the subquery, which is slow, but then it can do in-memory compares, which is fast, while the NOT EXISTS query can use matching indexes, which are faster than building a memory table ... As you can see, I am easily confused.
And what about the LEFT OUTER JOIN testing for IS NULL? A perfectly good alternate solution:
select c.client_name
from clients as c
left outer
join clientwarnings as cw
on c.client_id = cw.client_id
where cw.client_id is null
Answer 4 :
When coding a SQL statement with tables in master-detail relationships, it's common to have to decide whether to write the query using the WHERE EXISTS (. . .) clause or the WHERE value IN (. . .) clause. You may resist using WHERE EXISTS because it has the awkward syntax of returning a value, which you always ignore.
However, there's a difference when using rule-based optimization. You can determine the performance of a rule-based query by understanding which table is the driving table and how many rows each part returns.
When you write a query using the IN clause, you're telling the rule-based optimizer that you want the inner query to drive the outer query (think: IN = inside to outside). For example, to query the 14-row EMP table for the direct reports to the employee KING, you could write the following:
select ename from emp e
where mgr in (select empno from emp where ename = 'KING');
Here's the EXPLAIN PLAN for this query:
OBJECT OPERATION
---------- ----------------------------------------
SELECT STATEMENT()
NESTED LOOPS()
EMP TABLE ACCESS(FULL)
EMP TABLE ACCESS(BY INDEX ROWID)
PK_EMP INDEX(UNIQUE SCAN)
This query is virtually equivalent to this:
select e1.ename from emp e1,(select empno from emp where ename = 'KING') e2
where e1.mgr = e2.empno;
You can write the same query using EXISTS by moving the outer query column to a subquery condition, like this:
select ename from emp e
where exists (select 0 from emp where e.mgr = empno and ename = 'KING');
When you write EXISTS in a where clause, you're telling the optimizer that you want the outer query to be run first, using each value to fetch a value from the inner query (think: EXISTS = outside to inside).
The EXPLAIN PLAN result for the query is:
OBJECT OPERATION
---------- ----------------------------------------
SELECT STATEMENT()
FILTER()
EMP TABLE ACCESS(FULL)
EMP TABLE ACCESS(BY INDEX ROWID)
PK_EMP INDEX(UNIQUE SCAN)
This is virtually similar to the PL/SQL code:
set serveroutput on;
declare
l_count integer;
begin
for e in (select mgr,ename from emp) loop
select count(*) into l_count from emp
where e.mgr = empno and ename = 'KING';
if l_count != 0 then
dbms_output.put_line(e.ename);
end if;
end loop;
end;
To determine which clause offers better performance in rule-based optimization, consider how many rows the inner query will return in comparison to the outer query. In many cases, EXISTS is better because it requires you to specify a join condition, which can invoke an INDEX scan. However, IN is often better if the results of the subquery are very small. You usually want to run the query that returns the smaller set of results first.
Some people avoid the EXISTS clause because of the requirement to return a result from the query--even though the result is never used. Depending on personal style, people often use 'X,' 1, 0, or null. From looking at the EXPLAIN PLAN output, it appears that the optimizer throws out whatever value you enter and uses 0 all the time. Many developers get into the habit of always entering some constant value.
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