MySQL中Group By使用注意事项

Posted 2020-08-06

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)