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.dname
FROM dept d,emp e
WHERE 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.dname
FROM dept d
WHERE EXISTS(select e.deptno
FROM emp e
WHERE 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.

Annem Jyothsna

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