问题：card 表的 card_number 字段忘记加唯一约束，造成大量数据重复，需要去重。

1 测试数据准备

创建表1

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6CREATE TABLE `card` (

`card_id` bigint(20) NOT NULL AUTO_INCREMENT COMMENT 'ID',

`card_number` varchar(100) DEFAULT NULL COMMENT '卡号',

`other_field` varchar(100) DEFAULT NULL COMMENT '其它字段',

PRIMARY KEY (`card_id`)

) ENGINE=InnoDB AUTO_INCREMENT=142665 DEFAULT CHARSET=utf8;

新建存储过程，准备 10 万条数据1

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15DROP PROCEDURE IF EXISTS proc1;

DELIMITER $$

SET AUTOCOMMIT = 0$$

CREATE PROCEDURE proc1()

BEGIN

DECLARE v_cnt DECIMAL (10) DEFAULT 0 ;

dd:LOOP

INSERT INTO card (card_number, other_field) VALUES (UUID(), UUID());

COMMIT;

SET v_cnt = v_cnt+1 ;

IF v_cnt = 100000 THEN LEAVE dd;

END IF;

END LOOP dd ;

END;$$

DELIMITER ;

调用存储过程1call proc1;

执行完存储过程后检查数据条数1select count(card_id) from card;

构建 1 万条卡号重复的数据(大概10%重复)1

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4insert into card(card_number) select card_number from card limit 5000;

commit;

insert into card(card_number) select card_number from card limit 5000;

commit;

2 检测重复数据

查询重复的卡号及其重复条数1select card_number, count(card_number) as count from card group by card_number having count(card_number) > 1;

3 数据去重

3.1 方案一：离线执行

离线执行：在执行去重数据期间，其它程序不对 card 表做增删改操作。

思路：将所有数据导入一张结构与其一样的空的临时表，但临时表需先建立重复字段的唯一索引，然后导入数据时，忽略错误，达到去重的效果。最后修改表名即可。

新建临时表 card_temp1

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6CREATE TABLE `card_temp` (

`card_id` bigint(20) NOT NULL AUTO_INCREMENT COMMENT 'ID',

`card_number` varchar(100) DEFAULT NULL COMMENT '卡号',

`other_field` varchar(100) DEFAULT NULL COMMENT '其它字段',

PRIMARY KEY (`card_id`)

) ENGINE=InnoDB AUTO_INCREMENT=142665 DEFAULT CHARSET=utf8;

在临时表中为卡号添加唯一索引1alter table card_temp add unique index(card_number);

将 card 表的数据导入 card_temp 表，但重复的则忽略1

2insert ignore into card_temp(card_id, card_number, other_field) select card_id, card_number, other_field from card order by null;

commit;

TIPS：实际测试，50万条数据，40秒左右。若数据量达百万级以上，建议分批执行。

检查临时表数据条数1select count(card_id) from card_temp;

修改表名称1

2alter table card rename to card_old;

alter table card_temp rename to card;

确认无误后删除旧数据表1drop table card_old;

3.2 方案二：在线执行

在线执行：在执行去重数据期间，其它程序可以对 card 表做增删改操作。

3.2.1 通过中间表

思路：将需保留的重复的数据导入一张结构与其一样的空的临时表，但临时表需先建立重复字段的唯一索引，确保数据无误，然后通过比对原始表，删除多余的重复数据。最后修改原始表的索引即可。

新建临时表 card_temp1

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6CREATE TABLE `card_temp` (

`card_id` bigint(20) NOT NULL AUTO_INCREMENT COMMENT 'ID',

`card_number` varchar(100) DEFAULT NULL COMMENT '卡号',

`other_field` varchar(100) DEFAULT NULL COMMENT '其它字段',

PRIMARY KEY (`card_id`)

) ENGINE=InnoDB AUTO_INCREMENT=142665 DEFAULT CHARSET=utf8;

在临时表中为卡号添加唯一索引1alter table card_temp add unique index(card_number);

在 card 表中为 card_number 建立普通索引，为后面优化去重效率1alter table card add index (card_number);

检查语句的消耗1

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4explain insert into card_temp

select b.card_id, a.card_number, a.other_field

from card a inner join (select max(card_id) card_id from card group by card_number having count(card_id)>1 order by null) b

on a.card_id = b.card_id;

将 card 表需保留的重复数据导入 card_temp 表1

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5insert into card_temp

select b.card_id, a.card_number, a.other_field

from card a inner join (select max(card_id) card_id from card group by card_number having count(card_id)>1 order by null) b

on a.card_id = b.card_id;

commit;

检查删除语句的消耗1explain delete a.* from card a, card_temp b where b.card_number = a.card_number and a.card_id < b.card_id;

删除 card 表重复的数据1

2delete a.* from card a, card_temp b where b.card_number = a.card_number and a.card_id < b.card_id;

commit;

TIPS：实际测试，50万条数据，20秒左右。若数据量达百万级以上，建议分批执行。

在 card 表中为 card_number 删除普通索引，并建立唯一索引，防止数据再次重复1

2alter table card drop index card_number;

alter table card add unique index(card_number);

确认无误后删除临时表1drop table card_temp;

3.2.2 直接删除

即时统计并删除1

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18DELETE a

FROM

card a

JOIN (

SELECT

card_number,

count(card_number) AS count,

max(card_id) AS card_id

FROM

card

GROUP BY

card_number

HAVING

count(card_number) > 1

) b ON a.card_number = b.card_number

WHERE

a.card_id < b.card_id;

commit;