MySQL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)
查找所有重复数据源码
/* 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: 0SELECT * 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: 0SELECT * 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)
