MysqlgroupbytopN的问题_MySQL

Mysql group by top N的问题

在日常工作中,经常要查询分组的前几名.oracle中可以通过row_num来支持查询,mysql暂时不支持row_num.那么如何来完成这个需求呢?

例如：

表中的数据：

+--------+-------+-----+| Person | Group | Age |+--------+-------+-----+| Bob    | 1     | 32  || Jill   | 1     | 34  || Shawn  | 1     | 42  || Jake   | 2     | 29  || Paul   | 2     | 36  || Laura  | 2     | 39  |+--------+-------+-----+

期望的结果：

+--------+-------+-----+| Shawn  | 1     | 42  || Jill   | 1     | 34  || Laura  | 2     | 39  || Paul   | 2     | 36  |+--------+-------+-----+

方式一:借鉴oracle中row_num的思想,在sql中增加伪列.

set @num := 0, @group := '';select person, `group`, agefrom (   select person, `group`, age,      @num := if(@group = `group`, @num + 1, 1) as row_number,      @group := `group` as dummy  from mytable  order by `Group`, Age desc, person) as x where x.row_number <= 2;

方式二:利用关联子查询

SELECT a.person, a.group, a.age FROM person AS a WHERE (SELECT COUNT(*) FROM person AS b WHERE b.group = a.group AND b.age >= a.age) <= 2 ORDER BY a.group ASC, a.age DESC