Pages

Monday 5 March 2012

SQL Questions and Answers

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.

No comments:

Post a Comment