将2列相同的表与另一列连接

Question

我想加入在另一个表中共享同一个外键的2列

这里是表格：

country:
idcountry| countryname
 1         german
 2         america

目的地

id|fromcountry    |tocountry
1  1               2

我想要的结果：

id|fromc    |toc
1  german    america

Answer 1

使用简单的Left Join ans为国家/地区名称列提供别名：

SELECT d.idcountry as id, cf.CountryName as fromc, ct.CountryName as toc
FROM destination d
LEFT JOIN country cf ON d.fromcountry = cf.idcountry
LEFT JOIN country ct ON d.tocountry = ct.idcountry

Answer 2

对两个字段使用左外连接 像这样的东西

SELECT Dest.ID, CFrom.CountryName, CTo.CountryName
FROM Destination Dest 
LEFT OUTER JOIN Country CFrom ON Dest.FromCountry = CFrom.idcountry
LEFT OUTER JOIN Country CTo ON Dest.ToCountry = CTo.idcountry