Question

I’m trying to convert this SQL into a Linq2SQL query, however, I’m at the point of just dragging a stored procedure into the dbml and hoped someone could do a better job. When there are no records for the nested query, it is returning null.

SQL:

SELECT
    Table1.Field1 -
    ISNULL(
            (
            SELECT  
                SUM(Table2.Field1) 
            FROM Table2 
                INNER JOIN Table3 ON Table2.ID = Table3.Table2ID 
            WHERE Table3.Table1ID = Table1.ID
            )
            ,0)
FROM
    Table1
WHERE
    (Table1.ID = @ID)

Linq2SQL

     return (from q in db.Table1s
                    where q.ID == id
                    select q.Field1.GetValueOrDefault() - 
                            (from o in db.Table2s 
                             join r in db.Table3s on o.ID equals r.Table2ID.GetValueOrDefault(0)
                             where r.Table1ID == q.ID
                             select Convert.ToInt32(o.Field1.GetValueOrDefault(0))).Sum()
                    ).SingleOrDefault()

Can anyone do a better job.

Answer 1

A concise version using method syntax and lambda expressions:

from t1 in Table1s
select t1.Field1 -  Table3s.Where(x=>x.Table1ID == t1.ID).Sum(y=>y.Table2.Field1)

A more readable version using query syntax with a let statement:

from t1 in Table1s
let tmp = 
(
    from t3 in Table3s
    where t3.Table1ID == t1.ID
    select t3.Table2.Field1
).Sum()
select t1.Field1 -  tmp

I strongly recommend using a tool like LinqPad to design your queries, because it will show you the SQL that is generated. This allows you to tune your more expensive queries for better results.

Its also useful for quick prototyping and testing of short blocks of code.