mysql - 获取每组分组结果的前n条记录

Question

以下是最简单的示例，但任何解决方案都应该能够扩展到所需的 n 个顶级结果:

给定一个如下表，其中包含人员、组和年龄列，您如何获得每个组中年龄最大的 2 个人？(组内的关系不应产生更多结果，但会给出前 2 个按字母顺序排列)

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

Desired result set:

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

---

NOTE: This question builds on a previous one- Get records with max value for each group of grouped SQL results - for getting a single top row from each group, and which received a great MySQL-specific answer from @Bohemian:

select * 
from (select * from mytable order by `Group`, Age desc, Person) x
group by `Group`

希望能够以此为基础，尽管我不知道如何实现。

Answer 1

这是执行此操作的一种方法，即使用 UNION ALL (请参阅 SQL Fiddle with Demo )。这适用于两个组，如果您有两个以上的组，则需要指定组编号并为每个组添加查询:

(
  select *
  from mytable 
  where `group` = 1
  order by age desc
  LIMIT 2
)
UNION ALL
(
  select *
  from mytable 
  where `group` = 2
  order by age desc
  LIMIT 2
)

有多种方法可以执行此操作，请参阅本文以确定适合您情况的最佳路线:

http://www.xaprb.com/blog/2006/12/07/how-to-select-the-firstleastmax-row-per-group-in-sql/

编辑:

这可能也适合您，它为每条记录生成一个行号。使用上面链接中的示例，这将仅返回行号小于或等于 2 的记录:

select person, `group`, age
from 
(
   select person, `group`, age,
      (@num:=if(@group = `group`, @num +1, if(@group := `group`, 1, 1))) row_number 
  from test t
  CROSS JOIN (select @num:=0, @group:=null) c
  order by `Group`, Age desc, person
) as x 
where x.row_number <= 2;

参见Demo