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`, age from ( 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
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