Mysql 的rank 函數如何實現
表特徵：
mysql> select * from test;
+------+------+
| a | b |
+------+------+
| 1 | 20 |
| 1 | 21 |
| 1 | 24 |
| 2 | 20 |
| 2 | 32 |
| 2 | 14 |
+------+------+
6 rows in set (0.00 sec)
現在，我們以a分組，查詢b列最大的2個值。 這條sql要怎麼寫了？
1.創建表
Create Table: CREATE TABLE `sam` (
`a` int(11) DEFAULT NULL,
`b` int(11) DEFAULT NULL
) ENGINE=MyISAM DEFAULT CHARSET=utf8
2.插入模擬數據
INSERT INTO `sam` VALUES (1,10),(1,15),(1,20),(1,25),(2,20),(2,22),(2,33),(2,45);
+------+------+
| a | b |
+------+------+
| 1 | 10 |
| 1 | 15 |
| 1 | 20 |
| 1 | 25 |
| 2 | 20 |
| 2 | 22 |
| 2 | 33 |
| 2 | 45 |
+------+------+
3.SQL實現
select a,b,rownum,rank from
(select ff.a,ff.b,@rownum:=@rownum+1 rownum,if(@pa=ff.a,@rank:=@rank+1,@rank:=1) as rank,@pa:=ff.a
FROM
(select a,b from sam group by a,b order by a asc,b desc) ff,(select @rank:=0,@rownum:=0,@pa=null) tt) result
having rank <=2;
4.結果：
+------+------+--------+------+
| a | b | rownum | rank |
+------+------+--------+------+
| 1 | 25 | 1 | 1 |
| 1 | 20 | 2 | 2 |
| 2 | 45 | 5 | 1 |
| 2 | 33 | 6 | 2 |
+------+------+--------+------+
4 rows in set (0.00 sec)
在這裏 有一個錯誤 就是 @pa=null 這裏寫的是判斷 應該改成 @pa:=nul