Question

I have the following code to retrieve customer name, total (orders ), sum (order details) for reach customer in Northwind database. The problem with below code is that it raises an exception since a few customers dont have any entry in orders table.

I know using the query syntax (join) the exception can be avoided. I want to know if the same can be handled with the extension method syntax.

 var customerOrders = db.Customers
    .Select(c => new 
    { 
        CompanyName = c.CompanyName, 
        TotalOrders = c.Orders.Count(), 
        TotalQuantity = c.Orders
            .SelectMany(o => o.Order_Details).Sum(o=>o.Quantity) 
    });

Answer 1

I think the problem is that in SQL, the SUM function can return null and Ling-to-SQL is expecting an int. You can get around this by doing this:

  TotalQuantity = 
    (int?)c.Orders.SelectMany(o => o.Order_Details).Sum(o=>o.Quantity)

and then checking for null values.
I think this will also work

  TotalQuantity = 
    ((int?)c.Orders.SelectMany(o => o.Order_Details).Sum(o=>o.Quantity)) ?? 0

if you want it to default to 0.

The reason this happens is because the SQL that Linq-to-SQL generates results in a null value for the quantity column when there are no details. An easy way to determine what this issue is with this kind of error is to set db.Log to Console.Out and copy the generated SQL into SSMS and see what the results are. You’ll likely see that the generated SQL creates a left join which results in one or more null values in the Quantity column. Either that, or there will be an subquery that results in the null value.