Difference between sub query and nested query ?
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Answer Posted / lakshmi
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)
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