Writing SQL Subqueries in Having Clause | Essential SQL

You can use sub queries in you SQL’s HAVING clause to filter out groups of records. Just as the WHERE clause is used to filter rows of records, the HAVING clause is used to filter groups. Because of this, it becomes very useful in filtering on aggregate values such as averages, summations, and count.

The power of using a subquery in the HAVING clause is now you don’t have to hard-code values within the comparisons. You can rely on the subquery’s results to do so for you.