This section is from the "Practical PostgreSQL" book, by John Worsley and Joshua Drake. Also available from Amazon: Practical PostgreSQL.
When a Group By is performed, the parser logically collects rows that have the same value for a specified column and then it groups them together. The following groups all publ_names together and return the sum of the number of publishers with that name.
Example 4-22. Using the GROUP BY Clause
SELECT publ_name, COUNT(*) FROM publish GROUP BY publ_name; publ_name | count ------------------+------- ABC Books | 2 Children's Books | 1 ODG Books | 1 Reading Rainbow | 1 Young Readers | 1 (5 rows)
 
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