Sams Teach Yourself SQL™ in 10 Minutes, Third Edition

Lesson 10. Grouping Data

In this lesson, you'll learn how to group data so that 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 last lesson, 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 DLL01:

SELECT COUNT(*) AS num_prodsFROM ProductsWHERE vend_id = 'DLL01'; ...