GROUP BY clause
The GROUP BY clause identifies the grouping columns for the query. It is used to group together those rows in a table that have the same values in all the columns listed. The order in which the columns are listed does not matter. The outcome is to combine each set of rows having common values into one group row that represents all rows in the group. Use a GROUP BY to eliminate redundancy in the output and to compute aggregates that apply to the groups. Grouping columns must be declared when the query computes aggregates with standard functions such as SUM, AVG, and COUNT. For more information, see Aggregate functions.
Syntax
[ GROUP BY expression [, ...] | ALL | aggregation_extension ]
where aggregation_extension is one of the following:
GROUPING SETS ( () | aggregation_extension [, ...] ) | ROLLUP ( expr [, ...] ) | CUBE ( expr [, ...] )
Parameters
- expression
-
The list of columns or expressions must match the list of non-aggregate expressions in the select list of the query. For example, consider the following simple query.
select listid, eventid, sum(pricepaid) as revenue, count(qtysold) as numtix from sales group by listid, eventid order by 3, 4, 2, 1 limit 5; listid | eventid | revenue | numtix -------+---------+---------+-------- 89397 | 47 | 20.00 | 1 106590 | 76 | 20.00 | 1 124683 | 393 | 20.00 | 1 103037 | 403 | 20.00 | 1 147685 | 429 | 20.00 | 1 (5 rows)In this query, the select list consists of two aggregate expressions. The first uses the SUM function and the second uses the COUNT function. The remaining two columns, LISTID and EVENTID, must be declared as grouping columns.
Expressions in the GROUP BY clause can also reference the select list by using ordinal numbers. For example, the previous example could be abbreviated as follows.
select listid, eventid, sum(pricepaid) as revenue, count(qtysold) as numtix from sales group by 1,2 order by 3, 4, 2, 1 limit 5; listid | eventid | revenue | numtix -------+---------+---------+-------- 89397 | 47 | 20.00 | 1 106590 | 76 | 20.00 | 1 124683 | 393 | 20.00 | 1 103037 | 403 | 20.00 | 1 147685 | 429 | 20.00 | 1 (5 rows) - ALL
-
ALL indicates to group by all columns specified in the SELECT list except those that are aggregated. For example, consider the following query which groups by
col1andcol2without having to specify them individually in the GROUP BY clause. The columncol3is the argument of theSUMfunction and thus not grouped.SELECT col1, col2 sum(col3) FROM testtable GROUP BY ALLIf you EXCLUDE a column in the SELECT list, the GROUP BY ALL clause does not group the results based on that specific column.
SELECT * EXCLUDE col3 FROM testtable GROUP BY ALL - aggregation_extension
-
You can use the aggregation extensions GROUPING SETS, ROLLUP, and CUBE to perform the work of multiple GROUP BY operations in a single statement. For more information on aggregation extensions and related functions, see Aggregation extensions.
Examples
The following examples use the SALES table that contains columns: salesid, listid, sellerid, buyerid, eventid, dateid, qtysold, pricepaid, commission, and saletime. For more information about the SALES table, see Sample database.
The following example query groups by salesid and listid
without having to specify them individually in the GROUP BY clause.
The column qtysold is the argument of the SUM function and thus not grouped.
SELECT salesid, listid, sum(qtysold) FROM sales GROUP BY ALL;
salesid | listid | sum
--------+---------+------
33095 | 36572 | 2
88268 | 100813 | 4
110917 | 127048 | 1
...The following example query excludes several columns in the SELECT list, so GROUP BY ALL only groups salesid and listid.
SELECT * EXCLUDE sellerid, buyerid, eventid, dateid, qtysold, pricepaid, commission, saletime
FROM sales GROUP BY ALL;
salesid | listid
--------+---------
33095 | 36572
88268 | 100813
110917 | 127048
...