SQL - Interview questions

1. To see current user name

Sql> show user;

2. Change SQL prompt name

SQL> set sqlprompt “Start > “

Start >

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

EXPLANATION

===========

Using the pseudocolumn ROWID is the fastest way to access a row. ROWID represents a unique storage identification number for a single row in a table (Note: Two rows on different tables but stored in the same cluster may have the same rowid value)

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 recordsOdd 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.

Q- Can two different users own tables with the same name?

A- Yes, tables are unique by username.tablename.

NOTE: Problems will occur if synonyms are used to make table names the same name; private synonyms override public synonyms.

 Q- Is it possible to update another table with 2 concatenated columns?
A- Yes, the following update statement will work:
UPDATE tablename SET colname = columnX || columnY
WHERE colname = ¤t_value_for_this_column ;
NOTE: The result is truncated to 255 characters.

Q- How can a new view be created from two old views that have common field names?
A- Use aliases in the fields. For example:
CREATE VIEW x AS
SELECT tab1.fld1 afld1, tab2.fld1 bfld1
FROM tab1,tab2
WHERE tab1.fld1=tab2.fld1;

Q- Can columns be called TO and FROM?
A- These are reserved words and cannot be used to name database objects.

Q- Is it allowed to have % in the field name?
A- Allowed, Yes! Recommended, No! It can be confused with the wildcard symbol ‘%’ used in the LIKE clause.
 

Q- Can you index columns defined in views?

A-      No, view columns themselves cannot be indexed.

B-      

Q- Is it possible to prompt a user for data when running a SQL script?

A- Yes, prefix the column name with & or &&.

The following method can be used :

SELECT fld1,fld2

FROM tab1,tab2

WHERE key1=’&key1′;

Prompts for Enter value from key1: when data is entered this produces an old/new information listing which may be turned off with

SET VERIFY OFF

If && is used, the value is prompted for once and then used automatically if that value is used again during that SQL*Plus session.

 Q- How do you select the first 10 rows of a table?
A- Use system variable rownum in where clause. For example:
SELECT *
FROM table1
WHERE ROWNUM < 11;

Q- How do I return the first 10 values that occur most frequently?
A- CREATE VIEW v1 as:
SELECT name, count(*) num
FROM table
GROUP BY name;

sELECT name,num
FROM v1 a
WHERE 10 > (SELECT COUNT(*)
FROM v1
WHERE a.num < num)
ORDER BY num;