Joe Celko's SQL for Smarties, 3rd Edition

20.1. GROUP BY Clause

The GROUP BY clause is based on simple partitions. A partition of a set divides the set into subsets such that the union of the subsets returns the original set, and the intersection of the subsets is empty. Think of it as cutting up a pizza pie—each piece of pepperoni belongs to one and only one slice of pizza. When you get to the section on SQL-99 OLAP extensions, you will see “variations on a theme” in the ROLLUP and CUBE operators, but this is where it all starts.

The GROUP BY clause takes the result of the FROM and WHERE clauses, then puts the rows into groups defined as having the same values for the columns listed in the GROUP BY clause. Each group is reduced to a single row in the result table. This result table ...