mysql 排序 相除 mysql排序去重

今日做一个功能需求，从表中排序取出前十个且数据不一致的数据，如下列a表中的值有以下数据：

代码如下

mysql> select * from a;
+----+----------+
| id | user |
+----+----------+
| 1 | zhangsan |
| 2 | lisi |
| 3 | wangwu |
| 4 | zhangsan |
| 5 | zhaosi |
| 6 | wangwu |
| 7 | lisi |
| 8 | lisi |
| 9 | zhaosi |
+----+----------+
9 rows in set (0.00 sec)

我们需要取出id最大的前四位，且user不能一致的信息，按照以上信息以及需求，我们要搜索出结果为

zhaosi

lisi

wangwu

zhangsan

不能按照普通的做法，如：

代码如下

mysql> select * from a order by id desc limit 4;
+----+--------+
| id | user |
+----+--------+
| 9 | zhaosi |
| 8 | lisi |
| 7 | lisi |
| 6 | wangwu |
+----+--------+
4 rows in set (0.00 sec)

这样搜索出来的有重复值，得使用distinct关键字

代码如下

mysql> select distinct user from a order by id desc limit 4;
+----------+
| user |
+----------+
| zhaosi |
| wangwu |
| lisi |
| zhangsan |
+----------+
4 rows in set (0.00 sec)

其实应该是lisi与wangwu互换一下才是比较理想的，因为lisi最大的ID是8，而wangwu最大的ID是6，可能是lisi有一个ID为2导致的，我们把ID为2的删除，在来试试

代码如下

mysql> delete from a where id=2;
Query OK, 1 row affected (0.02 sec)
mysql> select * from a;
+----+----------+
| id | user |
+----+----------+
| 1 | zhangsan |
| 3 | wangwu |
| 4 | zhangsan |
| 5 | zhaosi |
| 6 | wangwu |
| 7 | lisi |
| 8 | lisi |
| 9 | zhaosi |
+----+----------+
8 rows in set (0.00 sec)
mysql> select distinct user from a order by id desc limit 4;
+----------+
| user |
+----------+
| lisi |
| zhaosi |
| wangwu |
| zhangsan |
+----------+
4 rows in set (0.00 sec)

结果正是由于前边有较低的ID记录影响了排序。

虽然这条语句能搜索正确的效果，但可能排序不是那么理想，也就是ID最大的前四位能搜索出来，但在这四位数据里并不是按照ID大小排序的。

例1测试数据

代码如下

/* 表结构 */
DROP TABLE IF EXISTS `t1`;
CREATE TABLE IF NOT EXISTS `t1`(
`id` INT(1) NOT NULL AUTO_INCREMENT,
`name` VARCHAR(20) NOT NULL,
`add` VARCHAR(20) NOT NULL,
PRIMARY KEY(`id`)
)Engine=InnoDB;
/* 插入测试数据 */
INSERT INTO `t1`(`name`,`add`) VALUES
('abc',"123"),
('abc',"123"),
('abc',"321"),
('abc',"123"),
('xzy',"123"),
('xzy',"456"),
('xzy',"456"),
('xzy',"456"),
('xzy',"789"),
('xzy',"987"),
('xzy',"789"),
('ijk',"147"),
('ijk',"147"),
('ijk',"852"),
('opq',"852"),
('opq',"963"),
('opq',"741"),
('tpk',"741"),
('tpk',"963"),
('tpk',"963"),
('wer',"546"),
('wer',"546"),
('once',"546");
SELECT * FROM `t1`;
+----+------+-----+
| id | name | add |
+----+------+-----+
|  1 | abc  | 123 |
|  2 | abc  | 123 |
|  3 | abc  | 321 |
|  4 | abc  | 123 |
|  5 | xzy  | 123 |
|  6 | xzy  | 456 |
|  7 | xzy  | 456 |
|  8 | xzy  | 456 |
|  9 | xzy  | 789 |
| 10 | xzy  | 987 |
| 11 | xzy  | 789 |
| 12 | ijk  | 147 |
| 13 | ijk  | 147 |
| 14 | ijk  | 852 |
| 15 | opq  | 852 |
| 16 | opq  | 963 |
| 17 | opq  | 741 |
| 18 | tpk  | 741 |
| 19 | tpk  | 963 |
| 20 | tpk  | 963 |
| 21 | wer  | 546 |
| 22 | wer  | 546 |
| 23 | once | 546 |
+----+------+-----+
rows in set (0.00 sec)

查找id最小的重复数据(只查找id字段)

代码如下

/* 查找id最小的重复数据(只查找id字段) */
SELECT DISTINCT MIN(`id`) AS `id`
FROM `t1`
GROUP BY `name`,`add`
HAVING COUNT(1) > 1;
+------+
| id   |
+------+
|    1 |
|   12 |
|   19 |
|   21 |
|    6 |
|    9 |
+------+
rows in set (0.00 sec)

查找所有重复数据

代码如下

/* 查找所有重复数据 */
SELECT `t1`.*
FROM `t1`,(
SELECT `name`,`add`
FROM `t1`
GROUP BY `name`,`add`
HAVING COUNT(1) > 1
) AS `t2`
WHERE `t1`.`name` = `t2`.`name`
AND `t1`.`add` = `t2`.`add`;
+----+------+-----+
| id | name | add |
+----+------+-----+
|  1 | abc  | 123 |
|  2 | abc  | 123 |
|  4 | abc  | 123 |
|  6 | xzy  | 456 |
|  7 | xzy  | 456 |
|  8 | xzy  | 456 |
|  9 | xzy  | 789 |
| 11 | xzy  | 789 |
| 12 | ijk  | 147 |
| 13 | ijk  | 147 |
| 19 | tpk  | 963 |
| 20 | tpk  | 963 |
| 21 | wer  | 546 |
| 22 | wer  | 546 |
+----+------+-----+
rows in set (0.00 sec)

查找除id最小的数据外的重复数据

代码如下

/* 查找除id最小的数据外的重复数据 */
SELECT `t1`.*
FROM `t1`,(
SELECT DISTINCT MIN(`id`) AS `id`,`name`,`add`
FROM `t1`
GROUP BY `name`,`add`
HAVING COUNT(1) > 1
) AS `t2`
WHERE `t1`.`name` = `t2`.`name`
AND `t1`.`add` = `t2`.`add`
AND `t1`.`id` <> `t2`.`id`;
+----+------+-----+
| id | name | add |
+----+------+-----+
|  2 | abc  | 123 |
|  4 | abc  | 123 |
|  7 | xzy  | 456 |
|  8 | xzy  | 456 |
| 11 | xzy  | 789 |
| 13 | ijk  | 147 |
| 20 | tpk  | 963 |
| 22 | wer  | 546 |
+----+------+-----+
rows in set (0.00 sec)

例2，表中没有主键(可唯一标识的字段)，或者主键并非数字类型(也可以删除重复数据，但效率上肯定比较慢)

例2测试数据

代码如下

/* 表结构 */
DROP TABLE IF EXISTS `noid`;
CREATE TABLE IF NOT EXISTS `noid`(
`pk` VARCHAR(20) NOT NULL COMMENT '字符串主键',
`name` VARCHAR(20) NOT NULL,
`add` VARCHAR(20) NOT NULL,
PRIMARY KEY(`pk`)
)Engine=InnoDB;
/* 测试数据，与上例一样的测试数据，只是主键变为字符串形式 */
INSERT INTO `noid`(`pk`,`name`,`add`) VALUES
('a','abc',"123"),
('b','abc',"123"),
('c','abc',"321"),
('d','abc',"123"),
('e','xzy',"123"),
('f','xzy',"456"),
('g','xzy',"456"),
('h','xzy',"456"),
('i','xzy',"789"),
('j','xzy',"987"),
('k','xzy',"789"),
('l','ijk',"147"),
('m','ijk',"147"),
('n','ijk',"852"),
('o','opq',"852"),
('p','opq',"963"),
('q','opq',"741"),
('r','tpk',"741"),
('s','tpk',"963"),
('t','tpk',"963"),
('u','wer',"546"),
('v','wer',"546"),
('w','once',"546");
SELECT * FROM `noid`;
+----+------+-----+
| pk | name | add |
+----+------+-----+
| a  | abc  | 123 |
| b  | abc  | 123 |
| c  | abc  | 321 |
| d  | abc  | 123 |
| e  | xzy  | 123 |
| f  | xzy  | 456 |
| g  | xzy  | 456 |
| h  | xzy  | 456 |
| i  | xzy  | 789 |
| j  | xzy  | 987 |
| k  | xzy  | 789 |
| l  | ijk  | 147 |
| m  | ijk  | 147 |
| n  | ijk  | 852 |
| o  | opq  | 852 |
| p  | opq  | 963 |
| q  | opq  | 741 |
| r  | tpk  | 741 |
| s  | tpk  | 963 |
| t  | tpk  | 963 |
| u  | wer  | 546 |
| v  | wer  | 546 |
| w  | once | 546 |
+----+------+-----+
rows in set (0.00 sec)

为表添加自增长的id字段

代码如下

/* 为表添加自增长的id字段 */
ALTER TABLE `noid` ADD `id` INT(1) NOT NULL AUTO_INCREMENT, ADD INDEX `id`(`id`);
Query OK, 23 rows affected (0.16 sec)
Records: 23  Duplicates: 0  Warnings: 0
SELECT * FROM `noid`;
+----+------+-----+----+
| pk | name | add | id |
+----+------+-----+----+
| a  | abc  | 123 |  1 |
| b  | abc  | 123 |  2 |
| c  | abc  | 321 |  3 |
| d  | abc  | 123 |  4 |
| e  | xzy  | 123 |  5 |
| f  | xzy  | 456 |  6 |
| g  | xzy  | 456 |  7 |
| h  | xzy  | 456 |  8 |
| i  | xzy  | 789 |  9 |
| j  | xzy  | 987 | 10 |
| k  | xzy  | 789 | 11 |
| l  | ijk  | 147 | 12 |
| m  | ijk  | 147 | 13 |
| n  | ijk  | 852 | 14 |
| o  | opq  | 852 | 15 |
| p  | opq  | 963 | 16 |
| q  | opq  | 741 | 17 |
| r  | tpk  | 741 | 18 |
| s  | tpk  | 963 | 19 |
| t  | tpk  | 963 | 20 |
| u  | wer  | 546 | 21 |
| v  | wer  | 546 | 22 |
| w  | once | 546 | 23 |
+----+------+-----+----+
rows in set (0.00 sec)

MySQL中必须是有索引的字段才可以使用AUTO_INCREMENT

删除重复数据与上例一样，记得删除完数据把id字段也删除了

删除重复数据，只保留一条数据

代码如下

/* 删除重复数据，只保留一条数据 */
DELETE FROM `noid`
USING `noid`,(
SELECT DISTINCT MIN(`id`) AS `id`,`name`,`add`
FROM `noid`
GROUP BY `name`,`add`
HAVING COUNT(1) > 1
) AS `t2`
WHERE `noid`.`name` = `t2`.`name`
AND `noid`.`add` = `t2`.`add`
AND `noid`.`id` <> `t2`.`id`;
Query OK, 8 rows affected (0.05 sec)
/* 删除id字段 */
ALTER TABLE `noid` DROP `id`;
Query OK, 15 rows affected (0.16 sec)
Records: 15  Duplicates: 0  Warnings: 0
SELECT * FROM `noid`;
+----+------+-----+
| pk | name | add |
+----+------+-----+
| a  | abc  | 123 |
| c  | abc  | 321 |
| e  | xzy  | 123 |
| f  | xzy  | 456 |
| i  | xzy  | 789 |
| j  | xzy  | 987 |
| l  | ijk  | 147 |
| n  | ijk  | 852 |
| o  | opq  | 852 |
| p  | opq  | 963 |
| q  | opq  | 741 |
| r  | tpk  | 741 |
| s  | tpk  | 963 |
| u  | wer  | 546 |
| w  | once | 546 |
+----+------+-----+
rows in set (0.00 sec)