MySQL取每组的前N条记录

一、对分组的记录取前N条记录：例子：取前 2条最大(小)的记录

 1 1.用子查询：
 2 SELECT * FROM right2 a  WHERE 2>
 3 (SELECT COUNT(*) FROM right2 b WHERE b.id=a.id AND b.account>a.account)
 4 ORDER BY a.id,a.account DESC
 5 2.用exists半连接：
 6 SELECT * FROM right2 a  WHERE EXISTS
 7 (SELECT COUNT(*) FROM right2 b WHERE b.id=a.id AND a.account<b.account HAVING COUNT(*)<2)
 8 ORDER BY a.id,a.account DESC
 9 同理可以取组内最小的N条记录：
10 SELECT * FROM right2 a  WHERE 2>
11 (SELECT COUNT(*) FROM right2 b WHERE b.id=a.id AND b.account<a.account)
12 ORDER BY a.id,a.account DESC
13 用exists：
14 SELECT * FROM right2 a  WHERE EXISTS
15 (SELECT COUNT(*) FROM right2 b WHERE b.id=a.id AND a.account>b.account HAVING COUNT(*)<2)
16 ORDER BY a.id,a.account DESC

如果取每组的最大(小)一条记录我常用：

1 select id,val from t b inner join(select * from t a where  order by val desc) a  on a.id=b.id group by a.id order  by id;

二.实例：取每组最大的前 N条

 1 create table t2 (
 2   id int primary key,
 3   gid char,
 4   col1 int,
 5   col2 int
 6 ) engine=innodb;
 7 insert into tx01 values
 8 (1,‘A‘,31,6),
 9 (2,‘B‘,25,83),
10 (3,‘C‘,76,21),
11 (4,‘D‘,63,56),
12 (5,‘E‘,3,17),
13 (6,‘A‘,29,97),
14 (7,‘B‘,88,63),
15 (8,‘C‘,16,22),
16 (9,‘D‘,25,43),
17 (10,‘E‘,45,28),
18 (11,‘A‘,2,78),
19 (12,‘B‘,30,79),
20 (13,‘C‘,96,73),
21 (14,‘D‘,37,40),
22 (15,‘E‘,14,86),
23 (16,‘A‘,32,67),
24 (17,‘B‘,84,38),
25 (18,‘C‘,27,9),
26 (19,‘D‘,31,21),
27 (20,‘E‘,80,63),
28 (21,‘A‘,89,9),
29 (22,‘B‘,15,22),
30 (23,‘C‘,46,84),
31 (24,‘D‘,54,79),
32 (25,‘E‘,85,64),
33 (26,‘A‘,87,13),
34 (27,‘B‘,40,45),
35 (28,‘C‘,34,90),
36 (29,‘D‘,63,8),
37 (30,‘E‘,66,40),
38 (31,‘A‘,83,49),
39 (32,‘B‘,4,90),
40 (33,‘C‘,81,7),
41 (34,‘D‘,11,12),
42 (35,‘E‘,85,10),
43 (36,‘A‘,39,75),
44 (37,‘B‘,22,39),
45 (38,‘C‘,76,67),
46 (39,‘D‘,20,11),
47 (40,‘E‘,81,36);
48 create table tx01 (
49   id int primary key,
50   gid char,
51   col1 int,
52   col2 int
53 ) engine=innodb;

取每组gid 最大的前N条记录：使用自连接或则半连接：

*N=1时：

自连接：降序排好后group by取每组最大的一条。

select * from (select * from t2 order by col2 desc)as a group by gid order by gid;

半连接方式：找不到比最大值还大的。

select * from t2 a where not exists(select 1 from t2 b where b.gid=a.gid and b.col2>a.col2) order by a.gid; 

*N=3时：

自连接：

select * from t2 a where 3>(select count(*) from t2 where gid=a.gid and col2>a.col2) order by a.gid,a.col2 desc;

半连接：

select * from t2 a where exists(select count(*) from t2 b where b.gid=a.gid and a.col2<b.col2 having(count(*))<3) order by a.gid,a.col2 desc

转：本文出自 http://huanghualiang.blog.51cto.com/6782683/1252630