Answer Posted / pankaj

In sql server,

WHEN YOU USE 'IN', WHILE CHECKING FOR WHERE CONDITION SQL SERVER ENGINE DOES WHOLE TABLE SCAN. IF YOU USE 'EXISTS' AS SOON AS ENGINE FINDS THE REQUIRED ROW IT WILL STOP EXECUTING QUERY AND GOING FURTHER SCANNING TABLE.

In Oracle,
EXISTS is very faster than IN (when the subquery results is very large)

IN is faster than EXISTS (when the subquery results is very small)

In Example -
select ename from emp
where mgr in (select empno from emp where ename = 'KING');

Exists Example -
Select ename from emp a where exists ( select 0/null from emp b where a.mgr=b.empno and ename='KING')

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