使用左外连接的多个字段的LINQ连接查询

时间:2010-10-22 07:58:50

标签: c# .net linq

我有Product表和Officer表如下:

产品

ProductID | ProductName | Officer1ID | Officer2ID | Officer3ID
--------- | ----------- | ---------- | ---------- | ----------
12        | Mouse       | 123        | 124        | 125
13        | Keyboard    | 234        | 235        | 0

OfficerID | OfficerName 
--------- | ----------- 
123       | John       
124       | Andy    
125       | Mark



我需要从Product表中加入3列(Officer1ID,Officer2ID,Officer3ID)和Officer表中的OfficerID来生成如下结果:

ProductID | ProductName | Officer1Name | Officer2Name | Officer3Name
--------- | ----------- | ------------ | ------------ | ------------
12        | Mouse       | John         | Andy         | Mark
13        | Keyboard    | Dave         | Fred         | Leon



这是我的尝试。我知道如何加入1个字段,但不是多个字段。有人可以帮忙吗?谢谢!

List<Product> lstProduct = GetProducts();

List<Officer> lstOfficer = GetOfficers();

var merge = from p in lstProduct
   join from o in lstOfficers on p.Officer1ID equals o.OfficerID
   select new { ProductID = p.ProductID, ProductName = p.ProductName, OfficerName = o.OfficerName };

修改
Product表中的OfficerID可以为0(在Officer表中不存在)。

3 个答案:

答案 0 :(得分:4)

只需应用连接3次(每次OfficerID一次):

var merge = from p in lstProduct
            join o1 in lstOfficer on p.Officer1ID equals o1.OfficerID
            join o2 in lstOfficer on p.Officer2ID equals o2.OfficerID
            join o3 in lstOfficer on p.Officer3ID equals o3.OfficerID
            select new
                     {
                         ProductID = p.ProductID,
                         ProductName = p.ProductName,
                         Officer1Name = o1.OfficerName,
                         Officer2Name = o2.OfficerName,
                         Officer3Name = o3.OfficerName
                     };

答案 1 :(得分:3)

您可以使用multiple joins执行此操作。

您应该重新考虑您的数据模型。我建议使用junction table建立多对多关系:

<强>产品

ProductID | ProductName | ProductOfficiersID
--------- | ----------- | ---------- 
12        | Mouse       | 1        
13        | Keyboard    | 2       

<强> ProductOfficiers

ProductOfficiersID | ProductID | OficierId 
------------------ | --------- | -----------
1                  | 12        | 123     
1                  | 12        | 124  
1                  | 12        | 125
2                  | 13        | 234
...

<强>官 ...

答案 2 :(得分:0)

我最终使用子查询,灵感来自this thread

var merge = from p in lstProduct
            select new
            {
                p.ProductID,
                p.ProductName,
                Officer1Name = (from o in lstOfficer
                               where o.OfficerID == p.Officer1ID
                               select o.OfficerName).FirstOrDefault(),
                Officer1Name = (from o in lstOfficer
                               where o.OfficerID == p.Officer2ID
                               select o.OfficerName).FirstOrDefault(),
                Officer2Name = (from o in lstOfficer
                               where o.OfficerID == p.Officer3ID
                               select o.OfficerName).FirstOrDefault()
            };

谢谢你们的帮助!