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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).
GROUP BY ALL
Use GROUP BY ALL
to GROUP BY
all columns in the SELECT
statement that are not wrapped in aggregate functions.
This simplifies the syntax by allowing the columns list to be maintained in a single location, and prevents bugs by keeping the SELECT
granularity aligned to the GROUP BY
granularity (Ex: Prevents any duplication).
See examples below and additional examples in the “Friendlier SQL with DuckDB” blog post.
Multiple Dimensions
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.
Examples
Count the number of entries in the addresses
table that belong to each different city:
SELECT city, count(*)
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;
GROUP BY ALL
Examples
Group by city and street_name to remove any duplicate values:
SELECT city, street_name
FROM addresses
GROUP BY ALL;
Compute the average income per city per street_name. Since income is wrapped in an aggregate function, do not include it in the GROUP BY
:
SELECT city, street_name, avg(income)
FROM addresses
GROUP BY ALL;
-- GROUP BY city, street_name: