如果想要查询某个字段的分组，并且把分组里最大值及其对应的记录信息查询出来，一般会用GROUP BY和max一块用，但是这里面有一个坑。
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)新版本Mysql直接无法执行，除非设置sql_mode=only_full_group_by。
#1055 - Expression #1 of SELECT list is not in GROUP BY clause and contains nonaggregated column 'test.test.id' which is not functionally dependent on columns in GROUP BY clause; this is incompatible with sql_mode=only_full_group_by $ set sql_mode=' ' # 查找当前sql_mode SELECT @@sql_mode; # 查询的一般结果： # ONLY_FULL_GROUP_BY,STRICT_TRANS_TABLES,NO_ZERO_IN_DATE,NO_ZERO_DATE,ERROR_FOR_DIVISION_BY_ZERO,NO_AUTO_CREATE_USER,NO_ENGINE_SUBSTITUTION # 全局设置sql_mode(对新建的库生效) SET @@global.sql_mode ='STRICT_TRANS_TABLES,NO_ZERO_IN_DATE,NO_ZERO_DATE,ERROR_FOR_DIVISION_BY_ZERO,NO_AUTO_CREATE_USER,NO_ENGINE_SUBSTITUTION'; # 对已建好的库生效 SET sql_mode ='STRICT_TRANS_TABLES,NO_ZERO_IN_DATE,NO_ZERO_DATE,ERROR_FOR_DIVISION_BY_ZERO,NO_AUTO_CREATE_USER,NO_ENGINE_SUBSTITUTION';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，先排序，后分组，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)另外一种方法 (推荐) ： 就是先查出最大的值，然后根据这个值来查找对应的记录。
select * from test t where t.age = ( select max(age) from test where t.class = class ) order by class;