Answer Posted / ssk

1. DENSE_RANK is an Analytical Function which doesn't leave
gaps between the records. Coming to Rank function it leaves
gaps between the records.

Eg:

(RANK) (DENSE_RANK)

Sno Marks Ranks Sno Marks Ranks
----- ----- ----- ----- ----- -----
1 100 1 1 100 1
2 100 1 2 100 1
3 200 3 3 200 2
4 300 4 4 300 3
5 300 4 5 300 3
6 400 6 6 400 4

2. Substr will displays the text from position to TO position.
Instr returns the position of the character.

Eg: SELECT SUBSTR('ORACLE',2,4) FROM DUAL;
O/P : RAC

SELECT INSTR('ORACLE IS A DATABASE','A',1,2) from dual;
O/P : 11

3. PSEUDO Column : A column which is not an actual column in
the table. Eg : ROWNUM,ROWID,SYSDATE,USER,UID..

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