Answer / sriram

Correlated subquery runs once for each row selected by the
outer query. It contains a reference to a value from the
row selected by the outer query.

Nested subquery runs only once for the entire nesting
(outer) query. It does not contain any reference to the
outer query row.

For example

Correlated Subquery:

select e1.empname e1.basicsal e1.deptno from emp e1 where
e1.basicsal (select max(basicsal) from emp e2 where
e2.deptno e1.deptno)

Nested Subquery:

select empname basicsal deptno from emp where (deptno
basicsal) in (select deptno max(basicsal) from emp group by
deptno)



Please let me know if I am wrong...............

Answer / sriram

Correlated subquery runs once for each row selected by the
outer query. It contains a reference to a value from the
row selected by the outer query.

Nested subquery runs only once for the entire nesting
(outer) query. It does not contain any reference to the
outer query row.

For example

Correlated Subquery:

select e1.empname e1.basicsal e1.deptno from emp e1 where
e1.basicsal (select max(basicsal) from emp e2 where
e2.deptno e1.deptno)

Nested Subquery:

select empname basicsal deptno from emp where (deptno
basicsal) in (select deptno max(basicsal) from emp group by
deptno)



Please let me know if I am wrong...............

Answer / praveen

if we want to select 100th hig no.
in sub-query we will write 100 conditions.
to avoid this we go for correlated sub-query

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