发布于 2017-06-02 02:03:51 | 243 次阅读 | 评论: 0 | 来源: 网友投递
Mysql关系型数据库管理系统
MySQL是一个开放源码的小型关联式数据库管理系统，开发者为瑞典MySQL AB公司。MySQL被广泛地应用在Internet上的中小型网站中。由于其体积小、速度快、总体拥有成本低，尤其是开放源码这一特点，许多中小型网站为了降低网站总体拥有成本而选择了MySQL作为网站数据库。
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;