Using the EXISTS Command in SQL

Using the EXISTS Command in SQL

Suppose you wanted to retrieve the department number (deptno) and name (dname) fields for employees in an emp table. One way to get the values is with this SQL query: 

SELECT distinct d.deptno,d.dnameFROM dept d,emp eWHERE d.deptno=e.deptno

The preceding query takes four milliseconds to execute (on my machine).

But if you write the query like this instead…

SELECT d.deptno,d.dnameFROM dept dWHERE EXISTS(select e.deptnoFROM emp eWHERE d.deptno=e.deptno);

…you’ll find that the query is much faster (about one millisecond on my machine).

Apparently, using the EXISTS command improves the performance considerably.

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