由于之前设计表考虑不周导致存在重复数据，需要建立唯一索引失败，所以需先删除重复数据！
本例中，需要建立用户和门禁的联合唯一索引
1.先查询所有的重复数据
SELECT
*
FROM
`user_accesscontrol_auth`
WHERE
( `user_name`, `access_control_id` ) IN ( SELECT `user_name`, `access_control_id` FROM `user_accesscontrol_auth` GROUP BY `user_name`, `access_control_id` HAVING COUNT( 1 ) > 1 );
以上查询都是重复的记录！
2. 将重复的条目中ID更大的所有条目都删除，只留ID最小的那一个。（29万数据量，912条重复记录）
DELETE FROM `user_accesscontrol_auth`
WHERE (`user_name`, `access_control_id`) IN (SELECT `user_name`, `access_control_id` FROM (SELECT * FROM `user_accesscontrol_auth`) a GROUP BY `user_name`, `access_control_id` HAVING COUNT(1) > 1)
AND `id` NOT IN (SELECT MIN(`id`) FROM (SELECT * FROM `user_accesscontrol_auth`) b GROUP BY `user_name`, `access_control_id` HAVING COUNT(1) > 1);
删除重复记录保留一条：https://mp.weixin.qq.com/s/4ua_YF-Jbc89SA75Z9tNgg
3.数据删除成功后可建立user_name 和 access_control_id的联合唯一索引