MySQL中MAX函数与Group By一起使用的注意事项(转)
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)
原文来自:http://blog.csdn.net/magicharvey/article/details/21372813?utm_source=tuicool&utm_medium=referral