MySQL 中查找重复数据，删除重复数据
数据库版本 Server version: 5.1.41-community-log MySQL Community Server (GPL)
例1，表中有主键(可唯一标识的字段)，且该字段为数字类型
例1测试数据
/* 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 |
+----+------+-----+
23 rows in set (0.00 sec)
查找id最小的重复数据(只查找id字段)
/* 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 |
+------+
6 rows in set (0.00 sec)
查找所有重复数据
/* 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 |
+----+------+-----+
14 rows in set (0.00 sec)
查找除id最小的数据外的重复数据
/* 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 |
+----+------+-----+
8 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
/* 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 |
+----+------+-----+
23 rows in set (0.00 sec)
为表添加自增长的id字段
/* 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 |
+----+------+-----+----+
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
/* 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 |
+----+------+-----+
15 rows in set (0.00 sec)
