mysql中group by的用法_MySQL中Group By使用注意事项

mysql> select * from test;

+----+-------+------+-------+

| id | name | age | class |

+----+-------+------+-------+

| 1 | qiu | 22 | 1 |

| 2 | liu | 42 | 1 |

| 4 | zheng | 20 | 2 |

| 3 | qian | 20 | 2 |

| 0 | wang | 11 | 3 |

| 6 | li | 33 | 3 |

+----+-------+------+-------+

6 rows in set (0.00 sec)

如果想找到每个class里面的最大的age，则需要使用group by和max。

如下的sql语句，则输出结果有错误：

mysql> select id,name,max(age),class from test group by class;

+----+-------+----------+-------+

| id | name | max(age) | class |

+----+-------+----------+-------+

| 1 | qiu | 42 | 1 |

| 4 | zheng | 20 | 2 |

| 0 | wang | 33 | 3 |

+----+-------+----------+-------+

3 rows in set (0.00 sec)

虽然找到的age是最大的age，但是与之匹配的用户信息却不是真实的信息，而是group by分组后的第一条记录的基本信息。

如果我使用以下的语句进行查找，则可以返回真实的结果。

mysql> select * from (

-> select * from test order by age desc) as b

-> group by class;

+----+-------+------+-------+

| id | name | age | class |

+----+-------+------+-------+

| 2 | liu | 42 | 1 |

| 4 | zheng | 20 | 2 |

| 6 | li| 33 | 3 |

+----+-------+------+-------+

3 rows in set (0.00 sec)

distinct本来就是针对 “多列”的，如果想让单个列“distinct”，用group by

mysql> select a,b from abc;

+------+------+

| a | b |

+------+------+

| 7 | 9 |

| 7 | 1 |

| 7 | 1 |

+------+------+

3 rows in set (0.00 sec)

mysql> select distinct a,b from abc;

+------+------+

| a | b |

+------+------+

| 7 | 9 |

| 7 | 1 |

+------+------+

2 rows in set (0.00 sec)

mysql> select a, max(b) from abc group by a;

+------+--------+

| a | max(b) |

+------+--------+

| 7 | 9 |

+------+--------+

1 row in set (0.01 sec)