代码：

select d.Name as Department,e.Name as Employee,e.Salary
from Department as d,Employee as e
where d.Id=e.DepartmentId 
      and d.Id in (select max(Salary),Employee.DepartmentId 
                   from Employee
                   group by DepartmentId);

报错：

Operand should contain 1 column(s)

原因：

在子查询中，查出的数据是有两个字段的，而where语句中，“ ... d.Id in ....”，只有一个字段进行查询。也就是说，前后的字段数量不匹配，在使用in谓词时，in左右两边的字段数量要相同。将上面的代码改成如下可以成功。

select d.Name as Department,e.Name as Employee,e.Salary
from Department as d,Employee as e
where d.Id=e.DepartmentId 
      and (e.Salary, d.Id) in (select max(Salary),Employee.DepartmentId 
                   from Employee
                   group by DepartmentId);