我需要从雇员表中选择主管和非主管的姓名。因此，我需要返回两个单独的列，一个用于主管的姓名，一个用于雇员的姓名。
为此，我试着像这样使用where exists

select concat(first_name, middle_name, last_name) as supervisor_name, concat(first_name, middle_name, ulast_name) as employee_name
from employees
where exists (select employee_name from employees where employees.id = department.supervisor_id);

我还尝试在两个select子句之间创建一个联合，如下所示：

select  concat (first_name, middle_name, last_name) as supervisor_name
from employees
where exists (select * from department where employees.id = department.supervisor_id)
union
select  concat (first_name, middle_name, last_name) as employee_name
from femployees
where exists (select * from department where employees.id != department.supervisor_id);

请注意，部门是另一个表，其中包含主管的ID号。
我曾搜索过是否可以使用某种check约束作为替代，但没有找到。我还尝试应用select distinct，试图“除”返回的值，但无法使其工作。
我也尝试过使用别名，但它返回字段中的first_name是不明确的。

select concat (first_name, middle_name, last_name) as supervisor_name, concat (first_name, middle_name, last_name) as employee_name
from employees
join employees as supervisor_name on department.supervisor_id = employees.id
join employees as supervisor_id on department.supervisor_id != employees.id;