Question

I am populating a class using Linq to SQL.

What I am trying to do is query my database, return two integer values and subtract the two values from each other, producing the result, but I can’t think of a smart way to do it.

What can I do in this case ?

If it is not clear, , then this psuedocode implementation should clarify what functionality I wish for :

DECLARE @currentVal INT, @previousVal INT
@currentVal = SELECT VALUE
FROM Table1
WHERE Date = CURRDATE()

@previousVal = SELECT VALUE
FROM Table1
WHERE Date = MIN(Date)

RETURN @currentVal - @previousVal

But in Linq to SQL, (from o in context.Table1 where Date = currentDate select Value), how can I subtract the other value from this? Is this possible?

Answer 1

I’d stick to having it as a broken out set of queries, because you can then test if the values were actually returned or not and handle the case where too many values are returned:

var currentValResults = (from row in rows 
                         where row.Date == DateTime.Now 
                         select row.Value)
                        .ToArray();

var previousValResults = (from row in rows 
                          let minDate = rows.Min(r => r.Date)
                          where row.Date == minDate 
                          select row.Value)
                         .ToArray();

if (currentValResults.Length == 1 && previousValResults.Length == 1)
{
    var diff = currentValResults[0] - previousValResults[0];
}
else
{
    // Error condition?
}

Putting it all into a giant linq statement makes too many assumptions (or at least, my implementation does).