Mysql 的rank 函数如何实现网文纠错

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zsc1980 2014-11-13 16:45:35

文章标签 mysql Create 如何 rank 最大的 文章分类 MySQL 数据库

表特征：
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)

现在，我们以a分组，查询b列最大的2个值。这条sql要怎么写了？

1.创建表
Create Table: CREATE TABLE `sam` (
  `a` int(11) DEFAULT NULL,
  `b` int(11) DEFAULT NULL
) ENGINE=MyISAM DEFAULT CHARSET=utf8

2.插入模拟数据
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实现
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.结果：
+------+------+--------+------+
| 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)