SQL Queries for Mere Mortals: A Hands-On Guide to Data Manipulation in SQL, Second Edition

Using Aggregate Functions in Filters

Because an aggregate function returns a single value, you can use it as part of a comparison predicate in a search condition. You have to place the aggregate function within a subquery, however, and then use the subquery as part of the comparison predicate. If you’re thinking that this sounds familiar, you’re right. In Chapter 11, you learned how to use a subquery as part of a search condition in a WHERE clause and an aggregate function within a subquery. So you already know, in a general sense, how to use an aggregate function to filter the data sent to a result set. Now let’s expand on that knowledge.

Using an aggregate function as part of a comparison predicate allows you to test the value of a value expression ...