GROUP BY子句用于对查询结果进行分组，在GROUP BY中使用GROUPING SETS、CUBE、ROLLUP可以以不同的形式展示分组结果。

语法

GROUP BY expression [, ...]

GROUPING SETS

GROUPING SETS用于在同一结果集中指定多个GROUP BY选项，作用相当于多个GROUP BY查询的UNION组合形式。

SELECT origin_state, origin_zip, destination_state, sum(package_weight)
FROM shipping
GROUP BY GROUPING SETS (
    (origin_state),
    (origin_state, origin_zip),
    (destination_state));

上述示例等同于：

SELECT origin_state, NULL, NULL, sum(package_weight)
FROM shipping GROUP BY origin_state
UNION ALL
SELECT origin_state, origin_zip, NULL, sum(package_weight)
FROM shipping GROUP BY origin_state, origin_zip
UNION ALL
SELECT NULL, NULL, destination_state, sum(package_weight)
FROM shipping GROUP BY destination_state;

CUBE

CUBE用于列出所有可能的分组集。

SELECT origin_state, destination_state, sum(package_weight)
FROM shipping
GROUP BY origin_state, destination_state WITH CUBE

上述示例等同于：

SELECT origin_state, destination_state, sum(package_weight)
FROM shipping
GROUP BY GROUPING SETS (
    (origin_state, destination_state),
    (origin_state),
    (destination_state),
    ())

ROLLUP

ROLLUP可以以层级的方式列出分组集。

SELECT origin_state, origin_zip, sum(package_weight)
FROM shipping
GROUP BY ROLLUP (origin_state, origin_zip)

上述示例等同于：

SELECT origin_state, origin_zip, sum(package_weight)
FROM shipping
GROUP BY GROUPING SETS ((origin_state, origin_zip), (origin_state), ())

注意事项

• 查询中必须使用标准聚合函数（SUM、AVG或COUNT）声明非分组列，否则无法使用GROUP BY子句。

• GROUP BY中的列或表达式列表必须与查询列表中的非聚合表达式的列相同。

示例

例如，以下查询列表中包含两个聚合表达式，第一个聚合表达式使用SUM函数，第二个聚合表达式使用COUNT函数，其余两列（LISTID、EVENTID）声明为分组列。

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)

GROUP BY子句中的表达式也可以使用序号来引用所需的列。

例如，上述示例可改写为以下形式。

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