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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
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