Introduction to SQL: Mastering the Relational Database Language, Fourth Edition/20th Anniversary Edition

Chapter 10. SELECT Statement: The GROUP BY Clause

Introduction

The GROUP BY clause groups rows on the basis of similarities between them. You could, for example, group all the rows in the PLAYERS table on the basis of the place of residence; the result would be one group of players per town. From there you could query how many players there are in each group. The question that is actually answered is then: How many players live in each town? Other examples follow: How many matches have been played per team, and how much has been incurred in penalties per player? In short, the GROUP BY clause is frequently used to formulate questions based on the word per.

By adding aggregation functions, such as COUNT and SUM, to a select block with the use of a ...