經常看到問題，如何取出每組的前N條記錄。方便大家參考於是便把常見的幾種解法列出於下。
問題：有表 如下，要求取出各班前兩名（允許並列第二）
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 |
+----+------+------+-----+
結果如下
+----+------+------+-----+
| 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：（這個方法通用性較強）
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....
這裏列出了多種SQL語句的實現方法，有些是MySQL特有的(Limit, 其它數據庫可根據實際更改，比如oracle的rownum,MS SQL SERVER 的 top,..)，有時是SQL標準支持的。但效率上和應用的場合或許不同。具體應用時可根據實際表中的記錄情況，索引情況進行選擇。
特例 N=1 ,即取最大的/最小的一條記錄。
+----+------+------+-----+
| 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
個人轉發大神文，重複見諒。
