October 2012
Beginner
288 pages
4h 37m
English
Content preview from Sams Teach Yourself SQL in 10 Minutes, Fourth Edition
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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:
Input
SELECT COUNT(*) AS num_prods FROM Products WHERE vend_id ...
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