请参考:
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
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