GROUP BY
Contents
Use the GROUP BY clause in a SELECT statement to group rows together that have the same value in one or more column, or the same computed value using expressions with any
functions and operators except
grouping functions. When you
use a GROUP BY clause, you will get a single result row for each group of rows
that have the same value for the expression given in GROUP BY.
When grouping rows, grouping values are compared as if by the = operator.
For string values, the = operator ignores trailing whitespace and may normalize
characters and ignore case, depending on the collation in use.
You can use any of the grouping functions in your select expression. Their values will
be calculated based on all the rows that have been grouped together for each result
row. If you select a non-grouped column or a value computed from a non-grouped
column, it is undefined which row the returned value is taken from. This is not permitted if the ONLY_FULL_GROUP_BY SQL_MODE is used.
You can use multiple expressions in the GROUP BY clause, separated by commas.
Rows are grouped together if they match on each of the expressions.
You can also use a single integer as the grouping expression. If you use an integer n, the results will be grouped by the nth column in the select expression.
The WHERE clause is applied before the GROUP BY clause. It filters non-aggregated
rows before the rows are grouped together. To filter grouped rows based on aggregate values,
use the HAVING clause. The HAVING clause takes any expression and evaluates it as
a boolean, just like the WHERE clause. You can use grouping functions in the HAVING
clause. As with the select expression, if you reference non-grouped columns in the HAVING
clause, the behavior is undefined.
By default, if a GROUP BY clause is present, the rows in the output will be sorted by the expressions used in the GROUP BY. You can also specify ASC or DESC (ascending, descending) after those expressions, like in ORDER BY. The default is ASC.
If you want the rows to be sorted by another field, you can add an explicit ORDER BY. If you don't want the result to be ordered, you can add ORDER BY NULL.
WITH ROLLUP
The WITH ROLLUP modifer adds extra rows to the resultset that represent super-aggregate summaries. For a full description with examples, see SELECT WITH ROLLUP.
GROUP BY Examples
Consider the following table that records how many times each user has played and won a game:
CREATE TABLE plays (name VARCHAR(16), plays INT, wins INT); INSERT INTO plays VALUES ("John", 20, 5), ("Robert", 22, 8), ("Wanda", 32, 8), ("Susan", 17, 3);
Get a list of win counts along with a count:
SELECT wins, COUNT(*) FROM plays GROUP BY wins; +------+----------+ | wins | COUNT(*) | +------+----------+ | 3 | 1 | | 5 | 1 | | 8 | 2 | +------+----------+ 3 rows in set (0.00 sec)
The GROUP BY expression can be a computed value, and can refer back to an identifer
specified with AS. Get a list of win averages along with a count:
SELECT (wins / plays) AS winavg, COUNT(*) FROM plays GROUP BY winavg; +--------+----------+ | winavg | COUNT(*) | +--------+----------+ | 0.1765 | 1 | | 0.2500 | 2 | | 0.3636 | 1 | +--------+----------+ 3 rows in set (0.00 sec)
You can use any grouping function in the select expression. For each win average as above, get a list of the average play count taken to get that average:
SELECT (wins / plays) AS winavg, AVG(plays) FROM plays GROUP BY winavg; +--------+------------+ | winavg | AVG(plays) | +--------+------------+ | 0.1765 | 17.0000 | | 0.2500 | 26.0000 | | 0.3636 | 22.0000 | +--------+------------+ 3 rows in set (0.00 sec)
You can filter on aggregate information using the HAVING clause. The HAVING
clause is applied after GROUP BY and allows you to filter on aggregate data that is
not available to the WHERE clause. Restrict the above example to results that involve
an average number of plays over 20:
SELECT (wins / plays) AS winavg, AVG(plays) FROM plays GROUP BY winavg HAVING AVG(plays) > 20; +--------+------------+ | winavg | AVG(plays) | +--------+------------+ | 0.2500 | 26.0000 | | 0.3636 | 22.0000 | +--------+------------+ 2 rows in set (0.00 sec)
