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删除重复记录的sql语句
我们提供了两款删除重复记录的sql语句，一种是用id not in来实例，也算是联合查询的方法吧，第二种是临时表来删除重复记录。

1),delete from tablename where id not in (select max(id) from tablename group by col1,col2,...)

2),select distinct * into temp from tablename
  delete from tablename
  insert into tablename select * from temp

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