更改MYSQL中选定结果表中的列

Question

在这里，我想加入表' responsibilities '与字段Name, Direct, Supervise ：

Name | Direct | Supervise
 ABC    2        4

和positionCode，positionID的表'position'：

positionCode | positionID
   HR/HRM         2
   HR/MN          4

选定的结果表将是这样的事情。

Name | Direct | Supervise
 ABC   HR/HRM    HR/MN

在“ Direct ”和“ Supervise ”栏应该是positionCode从“位置”表。 是否有一体化查询输出此结果？ 或者我要查询2次？

Answer 1

试试这个查询，

SELECT r.Name,
       p1.positionCode AS Direct,
       p2.positionCode AS Supervise
FROM responsibilities r
LEFT JOIN positions p1
    ON r.Direct = p1.positionID
LEFT JOIN positions p2
    ON r.Supervise = p2.positionID

输出： SEE SQLFiddle DEMO

Answer 2

我想你可以加入responsibilities两次到positions表：

SELECT r.Name,
       COALESCE(p1.positionCode, 'Direct is N/A') AS Direct,
       COALESCE(p2.positionCode, 'Supervise is N/A') AS Supervise
FROM responsibilities r
LEFT JOIN positions p1
    ON r.Direct = p1.positionID
LEFT JOIN positions p2
    ON r.Supervise = p2.positionID

请点击以下链接查看正在运行的演示：

SQLFiddle

Answer 3

尝试以下查询，它应该工作

select R.Name,(select P.positionCode where R.Direct=P.positionID) as 
Direct,(select P.positionCode where R.Supervise=P.positionID) as 
Supervise from Responsibilites R, Positions P;

Answer 4

查看这个SQL Fiddle以证明此查询有效：

http://sqlfiddle.com/#!9/13845c/4/0

基本上，查询如下所示：

SELECT r.Name, p1.positionCode As Direct, p2.positionCode as Supervise
FROM responsibilities r 
LEFT JOIN positions p1 ON r.Direct = p1.positionID
LEFT JOIN positions p2 ON r.Supervise = p2.positionID