NOT EXISTS
时间: 2024-04-26 16:23:16 浏览: 130
NOT EXISTS是一种SQL查询语句中的谓词,用于判断某个条件是否不成立。当在一个NOT EXISTS子查询中,如果内层查询的结果为空,那么外层的WHERE子句将返回真值,否则返回假值。举一个例子,假设我们要查询没有选修1号课程的学生的姓名,我们可以使用NOT EXISTS来实现这个查询。查询语句如下:
SELECT Sname FROM Student WHERE NOT EXISTS(SELECT * FROM SC WHERE Sno = Student.Sno AND Cno='1');同时,需要注意的是,NOT EXISTS与EXISTS不可以直接互换使用。只有带有IN谓词、比较运算符、ANY或ALL谓词的子查询可以用带有EXISTS谓词的子查询等价替换。<span class="em">1</span><span class="em">2</span><span class="em">3</span>
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