[mysql] 先按某字段分組再取每組中前N條記錄


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

請參考: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);


期望結果
1) N=1 取GID每組 COL2最大的記錄
    +----+------+------+------+
    | 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每組 COL2最大的3條記錄
    +----+------+------+------+
    | 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

 


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