The GROUP BY clause specifies which grouping columns should be used to perform any aggregations in the SELECT clause. If the GROUP BY clause is specified, the query is always an aggregate query, even if no aggregations are present in the SELECT clause.

When a GROUP BY clause is specified, all tuples that have matching data in the grouping columns (i.e. all tuples that belong to the same group) will be combined. The values of the grouping columns themselves are unchanged, and any other columns can be combined using an aggregate function (such as COUNT, SUM, AVG, etc).

Normally, the GROUP BY clause groups along a single dimension. Using the GROUPING SETS, CUBE or ROLLUP clauses it is possible to group along multiple dimensions. See the GROUPING SETS page for more information.


-- count the number of entries in the "addresses" table that belong to each different city
FROM addresses
GROUP BY city;
-- compute the average income per city per street_name
SELECT city, street_name, AVG(income)
FROM addresses
GROUP BY city, street_name;