[置顶] 【mysql 分组取前几条】 先按某字段分组再取每组中前N条记录

CREATE TABLE `t2` (
`id` int(11) NOT NULL,
`gid` char(1) DEFAULT NULL,
`col1` int(11) DEFAULT NULL,
`col2` int(11) DEFAULT NULL,
PRIMARY KEY (`id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8;

insert into t2 values
(1,'A',31,6),
(2,'B',25,83),
(3,'C',76,21),
(4,'D',63,56),
(5,'E',3,17),
(6,'A',29,97),
(7,'B',88,63),
(8,'C',16,22),
(9,'D',25,43),
(10,'E',45,28),
(11,'A',2,78),
(12,'B',30,79),
(13,'C',96,73),
(14,'D',37,40),
(15,'E',14,86),
(16,'A',32,67),
(17,'B',84,38),
(18,'C',27,9),
(19,'D',31,21),
(20,'E',80,63),
(21,'A',89,9),
(22,'B',15,22),
(23,'C',46,84),
(24,'D',54,79),
(25,'E',85,64),
(26,'A',87,13),
(27,'B',40,45),
(28,'C',34,90),
(29,'D',63,8),
(30,'E',66,40),
(31,'A',83,49),
(32,'B',4,90),
(33,'C',81,7),
(34,'D',11,12),
(35,'E',85,10),
(36,'A',39,75),
(37,'B',22,39),
(38,'C',76,67),
(39,'D',20,11),
(40,'E',81,36);

SELECT id,gid,col1,col2 from t2
ORDER BY gid,col2

1   A   31  6
21  A   89  9
26  A   87  13
31  A   83  49
16  A   32  67
36  A   39  75
11  A   2   78
6   A   29  97
22  B   15  22
17  B   84  38
37  B   22  39
27  B   40  45
7   B   88  63
12  B   30  79
2   B   25  83
32  B   4   90
33  C   81  7
18  C   27  9
3   C   76  21
8   C   16  22
38  C   76  67
13  C   96  73
23  C   46  84
28  C   34  90
29  D   63  8
39  D   20  11
34  D   11  12
19  D   31  21
14  D   37  40
9   D   25  43
4   D   63  56
24  D   54  79
35  E   85  10
5   E   3   17
10  E   45  28
40  E   81  36
30  E   66  40
20  E   80  63
25  E   85  64
15  E   14  86

SELECT a.id,a.gid,a.col1,a.col2 FROM t2 a
LEFT JOIN t2 b
ON a.gid=b.gid AND a.col2>=b.col2
GROUP BY a.id,a.gid,a.col1,a.col2
HAVING COUNT(b.id)<=1
ORDER BY a.gid,a.col2 desc

1   A   31  6
22  B   15  22
33  C   81  7
29  D   63  8
35  E   85  10

SELECT a.id,a.gid,a.col1,a.col2 FROM t2 a
WHERE 1>=(
SELECT COUNT(*) FROM t2 b
WHERE a.gid=b.gid AND a.col2>=b.col2)
ORDER BY a.gid,a.col2 desc

1   A   31  6
22  B   15  22
33  C   81  7
29  D   63  8
35  E   85  10

SELECT a.id,a.gid,a.col1,a.col2 FROM t2 a
LEFT JOIN t2 b
ON a.gid=b.gid AND a.col2<=b.col2
GROUP BY a.id,a.gid,a.col1,a.col2
HAVING COUNT(b.id)<=1
ORDER BY a.gid,a.col2 desc

6   A   29  97
32  B   4   90
28  C   34  90
24  D   54  79
15  E   14  86

SELECT a.id,a.gid,a.col1,a.col2 FROM t2 a
WHERE 1>=(
SELECT COUNT(*) FROM t2 b
WHERE a.gid=b.gid AND a.col2<=b.col2)
ORDER BY a.gid,a.col2 desc

6   A   29  97
32  B   4   90
28  C   34  90
24  D   54  79
15  E   14  86