DISTINCT in DBMS

DISTINCT Syntax

SELECT DISTINCT column1, column2,
FROM table name;

Consider a sample table EMP

DISTINCT on a single column 

select DISTINCT job from emp;

Output

4 rows selected.

The above query Returns unique values present in the job column i.e.  even if a value is appeared for more than once but it is displayed only for one time.

DISTINCT on more than one column 

Whenever DISTINCT is applied on the select statement which contains more than one column then the combination of values of all the columns is considered as a single value even if any of the column value is different in that row it is considered as a DISTINCT value

Example 

select DISTINCT job,sal
from emp;

o/p

6 rows selected.

The above query Returns 6 values because each value is considered as a key combination of two values and [ANALYST, 3000] combination would appear twice and hence it is eliminated during display.

Another example for DISTINCT 

select DISTINCT ename, job
from emp;

Output

6 rows selected

The above query results in all the seven rows in the table because even if job column contains duplicate values name column contain only unique values hence the key combination both ename and job is considered as one entity and all values become unique