Sub query

Thanks,Vidya Rani

Nested Sub Query :  This Query will be Executed only once , inner query is excuted first and result is sent to outer query for execution

 Ex: select * from emp where (deptno,basicsal) in (select deptno,max(basicsal) from emp group by deptno)
  where as

Co-related Sub Query : is executed for every row of Outer or main Query , outer query is executed first and result is sent to inner query for execution

      Ex: select * from emp e1 where e1.basicsal=(selcte max(basicsal) from emp e2 where e2.deptno=e1.deptno)



     

can you elaborate with example ?

Both the answers is same.

Hi Arjun,

A correlated Subquery runs for the rows selected from the outer query.
 It takes the value from the outer query
and execute the inner query for that value

example:

select * from Employee e
where e.deptno in(select d.deptno from dept d
where e.deptno = d.deptno);

in this query Employee table's deptno will be passed into the inner query(select deptno from dept d where e.deptno = d.deptno.
And the inner query will execute only for that value from the outer query.
That's why it is called correlated subquery

In Nested subquery the inner query runs only once and pass the result set to the outer query.

example:

select * from Employee e
where e.deptno in (select d.deptno from dept d);

Here the inner query (select d.deptno form dept d) will run first and fetches all the rows from the dept table
and the outer query will select only the records that has the matching deptno in the result set fetched by the
inner query. The outer query will act as a nesting query and that is why this is called nested subquery.

Here in correlated subquery, the outer query executes first and the inner query will execute second.

But in Nested subquery, the inner query executes first and the outer query executes second.

Hope this helps.

Thanks

Hi Arjun,


A correlated Subquery runs for the rows selected from the outer query. It takes the value from the outer query
and execute the inner query for that value

example:

select * from emp e
where e.deptno in(select d.deptno from dept d
where e.deptno = d.deptno);

in this query emp table's deptno will be passed into the inner query(select deptno from dept d where e.deptno = d.deptno#.
And the inner query will execute only for that value from the outer query.
That's why it is called correlated subquery

In Nested subquery the inner query runs only once and pass the result set to the outer query.

example

select * from emp e
where e.deptno in#select d.deptno from dept d#;

Here the inner query #select d.deptno form dept d) will run first and fetches all the rows from the dept table
and the outer query will select only the records that has the matching deptno in the result set fetched by the
inner query. The outer query will act as a nesting query and that is why this is called nested subquery.

Here in correlated subquery, the outer query executes first and the inner query will execute second.

But in Nested subquery, the inner query executes first and the outer query executes second.


Refer the link
http://en.allexperts.com/q/Oracle-1451/2009/7/Diff-Nested-Correlated-subqueries.htm


Thanks