Question

I have two objects, let’s call them Input and Output

Input has properties Input_ID, Label, and Input_Amt
Output has properties Output_ID and Output_Amt

I want to perform the equivalent SQL statement in LINQ:

SELECT Label, Sum(Added_Amount) as Amount FROM     (SELECT I.Label, I.Input_Amt + ISNULL(O.Output_Amt, 0) as Added_Amount     FROM Input I LEFT OUTER JOIN Output O ON I.Input_ID = O.Output_ID) GROUP BY Label 

For the inner query, I’m writing something like:

var InnerQuery = from i in input                  join o in output                  on i.Input_ID equals o.Output_ID into joined                  from leftjoin in joined.DefaultIfEmpty()                  select new                  {                      Label = i.Label,                      AddedAmount = (i.Input_Amt + leftjoin.Output_Amt)                  }; 

In testing, however, the statement returns null. What gives?

Also, how can I continue the desired query and perform the group after I’ve added my amounts together, all within a single LINQ statement?

Answer 1

Okay, now that I understand what’s going on a bit better, the main problem is that you haven’t got the equivalent of the ISNULL bit. Try this instead:

var InnerQuery = from i in input                  join o in output                  on i.Input_ID equals o.Output_ID into joined                  from leftjoin in joined.DefaultIfEmpty()                  select new                  {                      Label = i.Label,                      AddedAmount = (i.Input_Amt + (leftjoin == null ? 0 : leftjoin.Output_Amt))                  };