今天幫別人修改程序，需要分組去最大值。也就是所謂，某一字段有重復字，過濾掉有重復的數據。弄了半天，最后總結一下。

首先搞點模擬數據出來

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 |

+----+------+------+------+

過程不闡述了，得到的最優算法是SELECT gid,col2,1 INTO @i,@j,@mc FROM t2v LIMIT 1;

SELECT id,gid,col1,col2 FROM (

SELECT a.id,a.gid,a.col1,a.col2,

IF(@i<>gid,@mc:=1,@mc) AS a2,

IF(@i<>gid,@i:=gid,@i) AS b1,

IF(@i=gid AND col2>=@j,@mc:=@mc+1,@mc) AS a1

FROM t2v a

ORDER BY gid,col2 DESC) aa

WHERE a2<=3

如果還加上索引，這個語句的查詢效果還是非常給力的，該語句支持單一字段數據重復，或者多字段重復，并且支持重復數據顯示N條，例子語句為3條

不過后來發現，在PHP的函數mysql_query中悲劇了，不支持一次執行多條語句，簡單的很，數組搞定，如下

最后在來一個變通了一下SQL語句，一句話搞定，不錯，效率比上面的那個來稍微查一點。SELECT a.id,a.gid,a.col1,a.col2 FROM t2v a

LEFT JOIN t2v 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

總結

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