LINQ连接查询，使用左外部连接具有多个字段

Question

I have Product table and Officer table as follow:

Product

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

Officer

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

I need to join 3 columns (Officer1ID, Officer2ID, Officer3ID) from Product table with OfficerID in Officer table to produce result like this:

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

This is my attempt. I know how to join 1 field, but not multiple. Can anyone help? Thanks!

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 };

EDIT
OfficerIDs in Product table could be 0(not exist in Officer table).

Answer 1

Just apply the join 3 times (once for each OfficerID ):

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
                     };

Answer 2

You can do this using multiple joins .

You should reconsider your data model. I suggest using junction table to make many to many relationship:

Product

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

ProductOfficiers

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

Officer ...

Answer 3

I ended up using subquery, inspired from 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()
            };

Thanks guys for your help!