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