mysql 删除重复记录，保留最新的一条：
delete from wechat_exchange_address where open_id in (select open_id from (select open_id from wechat_exchange_address a group by a.open_id having count(a.open_id) > 1) as a) and id not in (select id from (select max( id) id from wechat_exchange_address group by open_id having count( open_id)>1) as b);
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