检查重复记录

-- 检查重复code1select count(identity) num, identity from event_log where code='code1' group by identity having count(identity) > 1order by num desc

删除重复记录

DELETE FROM event_log WHERE `code`='code1' AND identity IN (    SELECT identity from (        SELECT identity FROM event_log WHERE code='code1' GROUP BY identity HAVING count(identity) > 1    ) a) AND id NOT IN (    SELECT keepId FROM (        SELECT min(id) keepId FROM event_log WHERE code='code1' GROUP BY identity HAVING count(identity) > 1    ) b)

其中 a 和 b 两个中间表的作用是, 避免执行时出现  You can't specify target table 'xxxxx' for update in FROM clause 错误

 

分组按时间正序取第一条记录, 巧妙地使用了not exists

select d.* from t_charge d where not exists (select 1 from t_charge where user_id = d.user_id and created_at < d.created_at)

按时间倒序则是

select f.* from t_charge f where not exists (select 1 from t_charge where user_id = f.user_id and created_at > f.created_at)