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

GROUP BY expression [, ...]

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。

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。
重要 AnalyticDB MySQL版不支持GROUP BY ROLLUP ()后加列名。如果您想以层级方式实现GROUP BY查询的UNION组合，可以通过GROUP BY GROUPING SETS ((column1, column2), (column1), ()) ,column3;实现。 
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), ())

示例

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;
返回结果如下：
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。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;
返回结果如下：
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