删除重复的记录,并保存第一条记录或保存最后一条记录

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删除表:dgpage中Mail重复的记录并保留最后一条.
delete dgpage where id in ( select a.id from dgpage a, dgpage b where a.mail = b.mail and a.id > b.id )

删除表:dgpage中Mail重复的记录并保留最近一条.
delete dgpage where id in ( select a.id from dgpage a, dgpage b where a.mail = b.mail and a.id <b.id )