如何加入空值兩個表中引用的字段

Question

表T1（ 「P 01」）和T2（ 「DTM」）包含以下值

Table t1 
PO101(string)  Loop_Id (Int) 
Item_1    6  
Item_2    8 
--- 
Table t2 
DTM02(string)  Loop_Id(int) 
20141029   (null) 
20141029   6 
20141101   8 

這個查詢

var records = (from t1 in x12.InterchangeDataSet.Tables["PO1"].AsEnumerable() 
       join t2 in x12.InterchangeDataSet.Tables["DTM"].AsEnumerable() 
       on t1.Field<string>("Loop_Id") equals t2.Field<string>("Loop_Id") 
       select new{A = t1.Field<string>("PO101"),B=t2.Field<string>("DTM02")}); 

失敗與

base {System.SystemException} = 
{"Cannot cast DBNull.Value to type 'System.Int32'. Please use a nullable type."} 

因爲t2.Loop_id中的空值。

如果我在t2中刪除包含空值的行，它工作正常。

如何將Loop_Id強制轉換爲可空或避免加入表中任何一個爲null的行？

Answer 1

var records = (from t1 in x12.InterchangeDataSet.Tables["PO1"].AsEnumerable() 
       join t2 in x12.InterchangeDataSet.Tables["DTM"].AsEnumerable() 
       on t1.Field<string>("Loop_Id") 
       equals (t2.Field<string>("Loop_Id") == null 
        ? string.Emtpty : t2.Field<string>("Loop_Id")) 
       select new{A = t1.Field<string>("PO101"),B=t2.Field<string>("DTM02")}); 

我認爲它應該工作。請嘗試。

Answer 2

使用下面的查詢來處理null。

var records = (from t1 in x12.InterchangeDataSet.Tables["PO1"].AsEnumerable() 
       join t2 in x12.InterchangeDataSet.Tables["DTM"].AsEnumerable() 
       on t1.Field<int>("Loop_Id") equals t2.Field<int>("Loop_Id") 
       where t2.Field<int>("Loop_Id") != null 
       select new{A = t1.Field<string>("PO101"),B=t2.Field<string>("DTM02")}); 

Answer 3

下面是解

var records = (from t1 in x12.InterchangeDataSet.Tables["PO1"].AsEnumerable() 
          join t2 in x12.InterchangeDataSet.Tables["DTM"].AsEnumerable() on t1.Field<Nullable<int>>("Loop_Id") 
          equals (t2.Field<Nullable<int>>("Loop_Id")) 
          select new{A = t1.Field<string>("PO101"),B=t2.Field<string>("DTM02")} 
          );