Posted in: Oracle
Oracle
Interview Questions and Answers : SQL
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
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.
Oracle FAQ :
1.How to find the version of each oracle
product installed? $ORACLE_HOME/orainst/inspdver utility provides a list of all the oracle products installed on the server with their verion numbers
When you need to upload huge amount of data, everybody says, that it is better to disable the constraints to give better performance. But, what if afterwards enabling of constraints fail due to bad data. You can find the culprit records by using EXCEPTIONS clause of ALTER TABLE statement. For ex.
ALTER TABLE test ENABLE CONSTRAINT pk_test exceptions into exceptions.
where exceptions table can be created by running $ORACLE_HOME/rdbms/admin/utlexcpt.sql script.
Executing EXPLAIN PLAN and then selecting from plan_table is one way to get the execution plan of a sql statement. But for this you need syntax of both statements and patience to type all that stuff for each statement. Instead SQL*PLUS offers a nifty command to enable and disable the display of execution plan of each statement executed in that session. In addition to this it can display the statistics for each statement.
SET AUTOT[RACE] {OFF | ON | TRACE[ONLY]} [EXP[LAIN]] [STAT[ISTICS]]
thus SET
There is a variable ERRORLEVEL in NT. When you exits from sqlplus with a constant and check this variable in NT script, you can trap the error . For ex. IF ERRORLEVEL 1 PLUS33 will startup the sqlplus if youspecified EXIT 1 to get out from the sqlplus.
If you query DBA_EXTENTS based on segment_name equal to your table name you will see that the column FILE_ID will show you the database file where each extent is located. You can then use FILE_ID to query DBA_DATA_FILES to find the name of the datafile. Or you could do it all in one query using a join.
It is possible to measure time interval of upto 1/100th of a second inoracle. DBMS_UTILITY.GET_TIME function returns a number which increments every 100th of second. Keep in mind that this number can be negative as well. Thus it can only be used for measuring time intervals. Also in sys.v_$timer table the column hsecs contains a number which also incre,emts every 100th of a second. This number can also be negative
It becomes tricky to use an index on date columns for date ranges beacause as soon as you use trunc(), oops! there oes you index. So here is a way to use your index on date column for date ranges...
where datefield >= to_date(to_char(date1,’DD-MON-YYYY’)||'00:00:00','DD-MON-YYYYHH24:MI:SS')
and datefield <= to_date(to_char(date2,’DD-MON-YYYY’)||'23:59:59','DD-MON-YYYYHH24:MI:SS')
Use a negative increment and select from the sequence once, then reset the increment back to its correct setting. For example, this should reset the qcs_ranum sequence back 100:
alter sequence qcs_ranum increment by -100;
select qcs_ranum.nextval from dual;
alter sequence qcs_ranum increment by 1;
Formatting
Columns in both Static and Dynamic Reports
Have you ever had to run a report in which the
default names of the columns were just so weird that there was no way a user
could decipher what that column heading meant? If you have, and if
you've looked at Oracles "SQL*Plus User Reference" (no technical info
- just how to use SQL*Plus) then you've probably seen the column command.
Oracle does a good job at documenting it - they just don't play it up
enough. Well, that's what I'm here for!!! So let's look at an
example:
select
to_char(sysdate,'MM/DD/YYYY HH24:MI:SS')
|
from dual
|
/
|
TO_CHAR(SYSDATE,'MM/DD/YYYYHH24:MI:SS')
---------------------------------------------------------------------------
06/17/1998 14:18:24
I imagine that this output looks exciting to the average user. One way to get his/her attention is to dress up the SELECT using a simple column formatting command such as ....
col a_col heading "Date and
Time" format a30
|
select
to_char(sysdate,'MM/DD/YYYY HH24:MI:SS') a_col
|
from dual
|
/
|
------------------------------
06/17/1998 14:20:27
This looks good, but is rather run of the mill. If you have to run the same SELECT twice, but you want the heading of the column to change with the second run, then try the following example:
col a_col heading "Date and
Time -
|
select
to_char(sysdate,'MM/DD/YYYY HH24:MI:SS') a_col
|
from dual
|
/
|
col a_col heading "Date and
Time -
|
/
|
------------------------------
06/17/1998 14:22:12
Time and Date -
------------------------------
06/17/1998 14:22:13
This solution looks so much more appealing than having to retype the same SELECT just to give it a different title. Any other change to the title is possible here as well. Just type it in and let it execute!