Mysql一些复杂的sql语句(查询与删除重复的行)

1.查找重复的行

SELECT * FROM blog_user_relation a WHERE (a.account_instance_id,a.follow_account_instance_id) 
IN (SELECT account_instance_id,follow_account_instance_id FROM blog_user_relation GROUP BY account_instance_id, follow_account_instance_id HAVING
 COUNT(*) > 1)

2.删除重复的行(保留一条)

PS：因为mysql的delete，如果被删的表的where条件里有in，且in里面也有此表，那就删除不了。

/*创建个临时表*/
CREATE TABLE blog_user_relation_temp AS
(
 SELECT * FROM blog_user_relation a WHERE 
 (a.account_instance_id,a.follow_account_instance_id) 
 IN ( SELECT account_instance_id,follow_account_instance_id FROM blog_user_relation GROUP BY account_instance_id, follow_account_instance_id HAVING COUNT(*) > 1)
 AND 
 relation_id 
 NOT IN (SELECT MIN(relation_id) FROM blog_user_relation GROUP BY account_instance_id, follow_account_instance_id HAVING COUNT(*)>1));

/*删除数据*/
DELETE FROM `blog_user_relation` WHERE relation_id IN (SELECT relation_id FROM blog_user_relation_temp);

/*删除临时表*/
DROP TABLE blog_user_relation_temp;