Question

有两个表

员工：employee_id，employee_name，manager_id，department_id
部门：department_id，manager_id

我需要显示员工ID，员工名称，经理姓名和经理ID，但我不知道如何显示经理姓名。这是我所拥有的，即使它是非常错误的并且没有显示真实的经理姓名

SELECT e.last_name "Employee", e.employee_id "Emp#", e.last_name "Manager",      
       d.manager_id "Mgr#"
FROM employees e, 
     departments d;

Answer 1

使用关键字Join弄清楚了-仅需要两次使用一张表

SELECT e.last_name "Employee", e.employee_id "Emp#", e2.last_name "Manager", 
       e.manager_id "Mgr#"
       FROM employees e
       JOIN employees e2 ON e2.employee_id = e.manager_id;

或者没有关键字加入/使用简单加入

SELECT e.last_name "Employee", e.employee_id "Emp#", e2.last_name "Manager", 
   e.manager_id "Mgr#"
   FROM employees e, employees e2
   WHERE e2.employee_id = e.manager_id

Answer 2

您可以在manager_id上进行内部联接，以便仅保留匹配的行

 SELECT last_name as "Employee", employee_id as "Emp#", last_name as "Manager", 
   manager_id as "Mgr#"
   FROM employees as e1
   INNER JOIN departments as d2 
   ON e1.manager_id = d2.manager_id;

SQL从同一表列联接

2 个答案: