The GROUP BY clause divides the output of a table into groups. You can group rows by one or more column names, or by the results of computed columns.
If a WHERE clause and a GROUP BY clause are present, the WHERE clause must appear before the GROUP BY clause. A GROUP BY clause, if present, must always appear before a HAVING clause. If a HAVING clause is specified but a GROUP BY clause is not, a GROUP BY () clause is assumed.
HAVING clauses and WHERE clauses can both be used in a single query. Conditions in the HAVING clause logically restrict the rows of the result only after the groups have been constructed. Criteria in the WHERE clause are logically evaluated before the groups are constructed, and so save time.
Understanding which queries are valid and which are not can be difficult when the query involves a GROUP BY clause. This section describes a way to think about queries with GROUP BY so that you may understand the results and the validity of queries better.
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