在MySQL中加入同一个表？

问题描述

| 我有下表，每个员工都有一个经理，如果一个人是他自己的经理，而managerid字段为null，我想要want0ѭ，因为结果表名称是employee

emid name managerid
1    raj  null
2    ram   1
3    ravi   null
4    arvind  3
5    rithu   2

我想要的结果是

emid name managername
1    raj  raj
2    ram   raj
3    ravi   ravi
4    arvind  ravi
5    rithu   ram

    

解决方法

SELECT e1.emid,e1.name,COALESCE(e2.name,e1.name) AS managername
    FROM employee e1
        LEFT JOIN employee e2
            ON e1.managerid = e2.emid

    ,未测试：

select a.name as name,b.name as managername from employee a inner join employee b on a.managerid=b.emid

这只会产生实际有经理的员工。但是，这就是您如何对自己的表进行联接。     ,

select
      e1.emid,e1.name ) ManagerName
   from 
      employees e1
         left join employees e2
            on e1.managerid = e2.emid

    ,

SELECT emid,name,CASE WHEN s.managerid IS NULL THEN s.name ELSE m.name END managername
  FROM employees s
  LEFT JOIN employees m ON s.managerid = m.managerid

要么

SELECT emid,m.name managername
  FROM employees s
  LEFT JOIN employees m ON COALESCE(s.managerid,s.emid) = m.emid