Chapter 9. Aggregation and GROUP BY

The SQL construction GROUP BY is a SELECT statement clause that is designed to be used in conjunction with aggregation (discussed in Chapter 5) to group data of similar types. An aggregate function is one that extracts informationsuch as a COUNT of rows or an average, minimum, or maximumby operating on multiple rows. We first discuss using GROUP BY on one column, and then on two columns. Then, we look at how to use GROUP BY in conjunction with the ORDER BY, HAVING, and WHERE clauses. Finally, we discuss aggregation with subqueries and complexities that nulls present in aggregate functions and other queries. As we introduce the GROUP BY and HAVING, and expand on the ORDER BY (which has been introduced earlier) in this chapter, we first present a SELECT in modified BNF showing the GROUP BY, HAVING and ORDER BY, before we start the rest of the discussion.