Sams Teach Yourself SQL in 10 Minutes, Second Edition

Filtering Groups

In addition to being able to group data using GROUP BY, SQL also allows you to filter which groups to include and which to exclude. For example, you might want a list of all customers who have made at least two orders. To obtain this data you must filter based on the complete group, not on individual rows.

You've already seen the WHERE clause in action (that was introduced back in Lesson 4, “Filtering Data.” But WHERE does not work here because WHERE filters specific rows, not groups. As a matter of fact, WHERE has no idea what a group is.

So what do you use instead of WHERE? SQL provides yet another clause for this purpose: the HAVING clause. HAVING is very similar to WHERE. In fact, all types of WHERE clauses you learned about ...