如何在引用的字段中连接两个具有空值的表

Question

表t1（“PO1”）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

此查询

  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")});

失败

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

因为t2.Loop_id中的空值。

如果我在t2中删除包含空值的行，它可以正常工作。

如何将Loop_Id转换为可为空或避免加入任何一个表中为null的行？

Answer 1

这是解决方案

 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")}
                           );

Answer 2

  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")});

我认为它应该有效。请试试。

Answer 3

使用以下查询处理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")});