与mysql中的同一个表连接？

Question

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，则 managerid 字段为空，我想要 emid 名称和经理 id 因为结果表名称是员工

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

，我想要结果为

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

Answer 1

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

Answer 2

未经测试：

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

这只会产生真正有经理的员工。但这就是连接表本身的方式。

Answer 3

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

Answer 4

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

或者

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