not exists不能仅仅理解为差集
要看以谁为主表，
见如下示例，同样的表，主表不一样，结果不一样：
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select * from [left] lwhere not exists (select * from [right] rwhere l.name=r.name)-------------------------------下面这个例子充分说明,not exists不能单单理解为差集select * from [right] rwhere not exists (select * from [left] lwhere l.name=r.name)?
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