EXISTS & IN operator

Question

Can any body give the advantages of EXISTS operator over IN?

Answer 1

Advantages of EXISTS over IN Clause in a Query
If you wish to retrieve from the department table all the department numbers that have at least one employee assigned to them, write the query as:
select deptno, deptname
from dept
where deptno in
     ( select deptno
from emp) ;
This query will run a full table scan on both the emp and dept tables. Even if there was an index on the deptno column of emp table, the sub-query would not be using the index and, hence, performance would suffer.
We can rewrite the query as:
select deptno, deptname
from dept
where exists in
     ( select deptno
from emp
where dept.deptno = emp.deptno) ;
This query uses an existing deptno index on the emp table, making the query much faster. Thus, wherever possible use EXISTS in place of IN clause in a query.
I cannot show you the explain plan, since I don't have these table's in my development enviroment, and I don't wish to create them, since I am at client's location. But any one of you can try this and then we can debate on this further.