MySQL® Crash Course

Chapter 13. Grouping Data

In this chapter, you’ll learn how to group data so you can summarize subsets of table contents. This involves two new SELECT statement clauses: the GROUP BY clause and the HAVING clause.

Understanding Data Grouping

In the previous chapter, you learned that the SQL aggregate functions can be used to summarize data. This enables you to count rows, calculate sums and averages, and obtain high and low values without having to retrieve all the data.

All the calculations thus far were performed on all the data in a table or on data that matched a specific WHERE clause. As a reminder, the following example returns the number of products offered by vendor 1003:

• Input

SELECT COUNT(*) AS num_prods FROM products WHERE vend_id = 1003; ...