如何在SQL中减少两个关系到一个关系？

Question

我有一个表ContractTable三列这样

ContractID | CompanyID | ContractorID 
-----------+------------+---------- 
    2  | 37725 | 37730 

此表有关系，这种观点ViewNameList：

ID  | CompanyTitle 
-----------+-------------- 
37725  |  X 
37730  |  Y 

我不想得到结果和两个连接。有一种方法可以像一个关系一样获得结果吗？

ContractID | CompanyID | ContractorID 
-----------+------------+---------- 
    2  |  X  |  Y 

Answer 1

您需要JOIN视图两次

select y.ContractID,v1.CompanyTitle as CompanyID,v2.CompanyTitle as ContractorID 
from yourtable y 
Left join view v1 on y.CompanyID = v1.ID  
Left join view v2 on y.ContractorID = v2.ID  

如果你不想加入两次，然后试试这个

SELECT y.ContractID, 
     Max(CASE WHEN y.CompanyID = v.ID THEN v.CompanyTitle END) AS CompanyID, 
     Max(CASE WHEN y.ContractorID = v.ID THEN v.CompanyTitle END) AS ContractorID 
FROM yourtable y 
     JOIN [view] v 
     ON v.ID IN (y.CompanyID, y.ContractorID) 
GROUP BY y.ContractID 

Answer 2

如果连接是困扰着你，在你能做到这一点不任何连接。即：

SELECT t1.ContractID , 
     (SELECT CompanyTitle FROM ViewNameList t2 WHERE t2.ID = t1.CompanyID) CompanyID, 
     (SELECT CompanyTitle FROM ViewNameList t2 WHERE t2.ID = t1.ContractorID) ContractorID 
     FROM ContractTable t1; 

和当然，Linq中这样做是很容易：

var contracts = from c in db.Contracts 
       select new { 
        ContractID = c.ContractID, 
        CompanyID = c.Company.CompanyTitle, 
        ContractorID = c.Contractor.CompanyTitle 
       };