分组取前N记录
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