[mysql] 先按某字段分组再取每组中前N条记录

From: http://blog.chinaunix.net/uid-26729093-id-4294287.html

请参考:http://bbs.csdn.net/topics/330021260

create table t2 (
    id int primary key,
    gid    char,
    col1    int,
    col2    int
) engine=myisam;

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);

期望结果
1) N=1 取GID每组 COL2最大的记录
    +----+------+------+------+
    | id | gid  | col1 | col2 |
    +----+------+------+------+
    |  6 | A    |   29 |   97 |
    | 15 | E    |   14 |   86 |
    | 24 | D    |   54 |   79 |
    | 28 | C    |   34 |   90 |
    | 32 | B    |    4 |   90 |
    +----+------+------+------+
2) N=3 取GID每组 COL2最大的3条记录
    +----+------+------+------+
    | id | gid  | col1 | col2 |
    +----+------+------+------+
    |  6 | A    |   29 |   97 |
    | 11 | A    |    2 |   78 |
    | 36 | A    |   39 |   75 |
    | 32 | B    |    4 |   90 |
    |  2 | B    |   25 |   83 |
    | 12 | B    |   30 |   79 |
    | 28 | C    |   34 |   90 |
    | 23 | C    |   46 |   84 |
    | 13 | C    |   96 |   73 |
    | 24 | D    |   54 |   79 |
    |  4 | D    |   63 |   56 |
    |  9 | D    |   25 |   43 |
    | 15 | E    |   14 |   86 |
    | 25 | E    |   85 |   64 |
    | 20 | E    |   80 |   63 |
    +----+------+------+------+

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)<=3
ORDER BY a.gid,a.col2 desc

SELECT a.id,a.gid,a.col1,a.col2 FROM t2 a
WHERE 3>=(
SELECT COUNT(*) FROM t2 b
WHERE a.gid=b.gid AND a.col2<=b.col2)
ORDER BY a.gid,a.col2 desc