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.