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子查询返回的结果是一行(也可以是多列),这种子查询称为行子查询
常用的操作符 : = 、<> 、IN 、NOT IN
张无忌
select salary, managerid from emp where name = '张无忌';
select * from emp where (salary, managerid) = (select salary, managerid from emp where name = '张无忌');