Question

I’m writing a query that looks like this:

var TheOutput = (from x in MyDC.MyTable
                 where.....
                 select new MyModel()
                 {
                    MyPropID = (from a in MyDC.MyOtherTable
                                where....
                                select a.ElementID).SingleOrDefault(),

                    MyPropData = (from a in MyDC.MyOtherTable
                                  where a.ElementID == MyPropID
                                  select a.ElementData).SingleOrDefault(),
                 }

I’m filling up MyModel with several properties from the database. Two of these properties are filled by reading another table. At the moment, I first read MyPropID to see if there’s an element in the other table and then I read the other table again to get the data, regardless of whether or not an ID was retrieved.

How can I eliminate this second read if I know, from reading MyPropID and returning a null, that there’s no data that matches the where a.ElementID == MyPropID clause.

Thanks.

Answer 1

var TheOutput = (from x in MyDC.MyTable
                 where.....
                 let id = (from a in MyDC.MyOtherTable
                                where....
                                select a.ElementID).SingleOrDefault()
                 select new MyModel()
                 {
                    MyPropID = id,
                    MyPropData = (from a in MyDC.MyOtherTable
                                  where id != null && a.ElementID == id
                                  select a.ElementData).SingleOrDefault()
                 }

If your code would create a single SQL statement from this query I do not think checking for null would matter. If this query would result in multiple SQL statements it might.