SQL Select具有对单个表的多个引用

Question

我有一个包含多个关系表的SQL
DB。主表中的某些字段多次引用另一个表。例如，假设我有一个销售员数据库，负责多个州的销售。我的数据库具有State1，State2和State3的字段，所有字段都映射回一个States表。我一生都不知道如何编写查询以返回具有所有枚举状态的记录。如果我只需要一个State字段，我就会知道：

SELECT Master.Name, State.Enumeration AS 'State'
FROM MasterTable Master, StateTable State
WHERE Master.State1 = State.ID;

如何为我的所有“州”字段扩展此字段？

谢谢。

Answer 1

从每个唯一的联接返回一列到状态：

select m.Name, s1.Enumeration as State1, s2.Enumeration as State2, s3.Enumeration as State3
from MasterTable m
left join StateTable s1 on m.State1 = s1.ID
left join StateTable s2 on m.State2 = s2.ID
left join StateTable s3 on m.State3 = s3.ID

从3个联接中返回所有状态的1列：

select m.Name, ISNULL(s1.Enumeration + ',','') 
               + ISNULL(s2.Enumeration + ',','') 
               + ISNULL(s3.Enumeration,'') as Enumeration
from MasterTable m
left join StateTable s1 on m.State1 = s1.ID
left join StateTable s2 on m.State2 = s2.ID
left join StateTable s3 on m.State3 = s3.ID

还有列查询…

select m.Name,
 ISNULL((select Enumeration from StateTable where ID = m.State1),'') as State1,
 ISNULL((select Enumeration from StateTable where ID = m.State2),'') as State2,
 ISNULL((select Enumeration from StateTable where ID = m.State3),'') as State3
from MasterTable m