Oracle Interview Questions and Answers

What is SQL and where does it come from?

Structured Query Language (SQL) is a language that provides an interface to relational database systems. SQL was developed by IBM in the 1970s for use in System R, and is a de facto standard, as well as an ISO and ANSI standard. SQL is often pronounced SEQUEL.

In common usage SQL also encompasses DML (Data Manipulation Language), for INSERTs, UPDATEs, DELETEs and DDL (Data Definition Language), used for creating and modifying tables and other database structures.

The development of SQL is governed by standards. A major revision to the SQL standard was completed in 1992, called SQL2. SQL3 support object extensions and will be (partially?) implemented in Oracle8.

 

What are the difference between DDL, DML and DCL commands?

DDL is Data Definition Language statements. Some examples:

• CREATE - to create objects in the database
• ALTER - alters the structure of the database
• DROP - delete objects from the database
• TRUNCATE - remove all records from a table, including all spaces allocated for the records are removed
• COMMENT - add comments to the data dictionary
• GRANT - gives user's access privileges to database
• REVOKE - withdraw access privileges given with the GRANT command

DML is Data Manipulation Language statements. Some examples:

• SELECT - retrieve data from the a database
• INSERT - insert data into a table
• UPDATE - updates existing data within a table
• DELETE - deletes all records from a table, the space for the records remain
• CALL - call a PL/SQL or Java subprogram
• EXPLAIN PLAN - explain access path to data
• LOCK TABLE - control concurrency

DCL is Data Control Language statements. Some examples:

• COMMIT - save work done
• SAVEPOINT - identify a point in a transaction to which you can later roll back
• ROLLBACK - restore database to original since the last COMMIT
• SET TRANSACTION - Change transaction options like what rollback segment to use

1.      To see current user name
 Sql> show user;

2.      Change SQL prompt name
 SQL> set sqlprompt “Manimara > “
Manimara >
Manimara >

3.      Switch to DOS prompt
 SQL> host

4.      How do I eliminate the duplicate rows ?
 SQL> delete from table_name where rowid not in (select max(rowid) from table group by duplicate_values_field_name);
or
SQL> delete duplicate_values_field_name dv from table_name ta where rowid <(select min(rowid)  from table_name tb where ta.dv=tb.dv);
Example.
Table Emp
Empno Ename
101               Scott
102               Jiyo
103               Millor
104               Jiyo
105               Smith
delete ename from emp a where rowid < ( select min(rowid) from emp b where a.ename = b.ename);
The output like,
Empno Ename
101               Scott
102               Millor
103               Jiyo
104               Smith

5.      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

6.      Display the records between two range
select rownum, empno, ename  from emp  where  rowid in
 (select rowid from emp where rownum <=&upto
 minus
 select rowid from emp where rownum<&Start);
Enter value for upto: 10
Enter value for Start: 7

   ROWNUM     EMPNO ENAME
--------- --------- ----------
        1      7782 CLARK
        2      7788 SCOTT
        3      7839 KING
        4      7844 TURNER

7.      I know the nvl function only allows the same data type(ie. number or char or date Nvl(comm, 0)), if commission is null then the text “Not Applicable”  want to display, instead of blank space. How do I write the query?

 SQL> select nvl(to_char(comm.),'NA') from emp;

Output :

NVL(TO_CHAR(COMM),'NA')
-----------------------
NA
300
500
NA
1400
NA
NA

8.      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.

9.      Explicit Cursor attributes
 There are four cursor attributes used in Oracle
cursor_name%Found, cursor_name%NOTFOUND, cursor_name%ROWCOUNT, cursor_name%ISOPEN

10.  Implicit Cursor attributes
 Same as explicit cursor but prefixed by the word SQL

SQL%Found, SQL%NOTFOUND, SQL%ROWCOUNT, SQL%ISOPEN

Tips : 1. Here SQL%ISOPEN is false, because oracle automatically closed the implicit cursor after executing SQL statements.
       : 2.  All are  Boolean attributes.

11.  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);

Enter value for n: 2
      SAL
---------
     3700

12.  To view installed Oracle version information
 SQL> select banner from v$version;

13.  Display the number value in Words
 SQL> select sal, (to_char(to_date(sal,'j'), 'jsp'))
from emp;
the output like,

      SAL (TO_CHAR(TO_DATE(SAL,'J'),'JSP'))
--------- -----------------------------------------------------
      800 eight hundred
     1600 one thousand six hundred
     1250 one thousand two hundred fifty
If you want to add some text like,
Rs. Three Thousand only.
SQL> select sal  "Salary ",
 (' Rs. '|| (to_char(to_date(sal,'j'), 'Jsp'))|| ' only.'))
 "Sal in Words" from emp
/
Salary  Sal in Words
------- ------------------------------------------------------
    800  Rs. Eight Hundred only.
   1600  Rs. One Thousand Six Hundred only.
   1250  Rs. One Thousand Two Hundred Fifty only.

14.  Display Odd/ Even number of records
 Odd number of records:
select * from emp where (rowid,1) in (select rowid, mod(rownum,2) from emp);
1
3
5
Even number of records:
select * from emp where (rowid,0) in (select rowid, mod(rownum,2) from emp)
2
4
6

15.  Which date function returns number value?
 months_between

16.  Any three PL/SQL Exceptions?
 Too_many_rows, No_Data_Found, Value_Error, Zero_Error, Others

17.  What are PL/SQL Cursor Exceptions?
 Cursor_Already_Open, Invalid_Cursor

18.  Other way to replace query result null value with a text
 SQL> Set NULL ‘N/A’
to reset SQL> Set NULL ‘’

19.   What are the more common pseudo-columns?
 SYSDATE, USER , UID, CURVAL, NEXTVAL, ROWID, ROWNUM

20.   What is the output of SIGN function?
 1 for positive value,
0 for Zero,
-1 for Negative value.

21.  What is the maximum number of triggers, can apply to a single table?
 12 triggers.

21.  How would you determine the time zone under which a database was operating?

select DBTIMEZONE from dual;

22.  Explain the use of setting GLOBAL_NAMES equal to TRUE.

Setting GLOBAL_NAMES dictates how you might connect to a database. This variable is either TRUE or FALSE and if it is set to TRUE it enforces database links to have the same name as the remote database to which they are linking.

23.  What command would you use to encrypt a PL/SQL application?

WRAP

24.  Explain the difference between a FUNCTION, PROCEDURE and PACKAGE.

A function and procedure are the same in that they are intended to be a collection of PL/SQL code that carries a single task. While a procedure does not have to return any values to the calling application, a function will return a single value. A package on the other hand is a collection of functions and procedures that are grouped together based on their commonality to a business function or application.