分组取前N记录

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经常看到问题，如何取出每组的前N条记录。方便大家参考于是便把常见的几种解法列出于下。

问题：有表 如下，要求取出各班前两名（允许并列第二）

Table1

+----+------+------+-----+

| id |SName |ClsNo |Score|

+----+------+------+-----+

| 1 |AAAA | C1 | 67 |

| 2 |BBBB | C1 | 55 |

| 3 |CCCC | C1 | 67 |

| 4 |DDDD | C1 | 65 |

| 5 |EEEE | C1 | 95 |

| 6 |FFFF | C2 | 57 |

| 7 |GGGG | C2 | 87 |

| 8 |HHHH | C2 | 74 |

| 9 |IIII | C2 | 52 |

| 10 |JJJJ | C2 | 81 |

| 11 |KKKK | C2 | 67 |

| 12 |LLLL | C2 | 66 |

| 13 |MMMM | C2 | 63 |

| 14 |NNNN | C3 | 99 |

| 15 |OOOO | C3 | 50 |

| 16 |PPPP | C3 | 59 |

| 17 |QQQQ | C3 | 66 |

| 18 |RRRR | C3 | 76 |

| 19 |SSSS | C3 | 50 |

| 20 |TTTT | C3 | 50 |

| 21 |UUUU | C3 | 64 |

| 22 |VVVV | C3 | 74 |

+----+------+------+-----+

结果如下

+----+------+------+-----+

| id |SName |ClsNo |Score|

+----+------+------+-----+

| 5 |EEEE | C1 | 95 |

| 1 |AAAA | C1 | 67 |

| 3 |CCCC | C1 | 67 |

| 7 |GGGG | C2 | 87 |

| 10 |JJJJ | C2 | 81 |

| 14 |NNNN | C3 | 99 |

| 18 |RRRR | C3 | 76 |

+----+------+------+-----+

方法一：

select a.id,a.SName,a.ClsNo,a.Score

from Table1 a left join Table1 b on a.ClsNo=b.ClsNo and a.Score<b.Score

group by a.id,a.SName,a.ClsNo,a.Score

having count(b.id)<2

order by a.ClsNo,a.Score desc

方法二：

select *

from Table1 a

where 2>(select count(*) from Table1 where ClsNo=a.ClsNo and Score>a.Score)

order by a.ClsNo,a.Score desc

方法三：

select *

from Table1 a

where id in (select id from Table1 where ClsNo=a.ClsNo order by Score desc
limit 2)

order by a.ClsNo,a.Score desc

方法....

这里列出了多种SQL语句的实现方法，有些是MySQL特有的(Limit, 其它数据库可根据实际更改，比如oracle的rownum,MS
SQL SERVER 的 top,..)，有时是SQL标准支持的。但效率上和应用的场合或许不同。具体应用时可根据实际表中的记录情况，索引情况进行选择。

特例 N=1 ,即取最大的/最小的一条记录。

+----+------+------+-----+

| id |SName |ClsNo |Score|

+----+------+------+-----+

| 5 |EEEE | C1 | 95 |

| 7 |GGGG | C2 | 87 |

| 14 |NNNN | C3 | 99 |

+----+------+------+-----+

select *

from Table1 a

where not exists (select 1 from Table1 where ClsNo=a.ClsNo and Score>a.Score);

select a.*

from Table1 a inner join (select ClsNo, max(Score) as mScore from Table1 group by ClsNo) b

on a.ClsNo=b.ClsNo and a.Score=b.Score

select *

from (select * from Table1 order by Score desc) t

group by ClsNo