How to substitute and display "0" for NULL when Query fails - sql

Question

I am working in MS WebMatrix using razor.
The issue I am having is that if one of my queries resolves to an error (dividing by 0, for instance) the whole page fails. I want the query to output "0" for this.
Code:
var itot = db.QueryValue("SELECT SUM(SUBTOTAL) from dbo.DR_TRANS where TRANSDATE = 41195 and SUBTOTAL >0");
var ctot = db.QueryValue("SELECT SUM(SUBTOTAL) from dbo.DR_TRANS where TRANSDATE = 41195 and TRANSTYPE = 1 and SUBTOTAL <0");
var nett = itot + ctot;
So, if either itot or ctot renders Null or error because there is no entries for the selected date, how do I get it to treat error as "0" so the math still works? In this example, itot is total invoices raised today, ctot is total credits raised today.
If there are no invoices yet, or if there are no credits raised yet, my code throws an error and shuts the page down. I want it to display 0 if there's none of either, and the value of itot if theres no ctot.
What am I doing wrong? it works fine the moment there's some of each, but I want it to work before that, because there's other code on the page for displaying other query results.
I'm very new to coding queries, so there will be a basic answer I have missed in my online searching!

Answer 1

Wrapping the aggregate function with COALESCE should return 0 rather than NULL:
SELECT COALESCE(SUM(SUBTOTAL), 0)
FROM dbo.DR_TRANS
WHERE TRANSDATE = 41195
AND SUBTOTAL >0
ie a function that returns the first parameter if it is not null, and the second parameter if the first is NULL. I don't know what function to use with webmatrix, but COALESCE is pretty standard. In Oracle, you could use NVL. ISNULL is another such function.

Answer 2

#Glenn offers a perfectly fine solution using SQL. However, you might prefer to understand how to deal with nulls in c#. The null-coalescing operator ?? which is a mouthful, says if something is not null, use it, and if it's null, use what's after the ??. An example is way easier to understand than what I just wrote. The last line of your code would look like:
var nett = itot ?? 0 + ctot ?? 0;
Basically the ?? 0 says if it's null, treat it as a zero, otherwise use the value itself.
I prefer to do as much as possible in c#, and keep my SQL as simple as possible. But I recognize that's more of a personal preference.
In general, there will be cases where you have to deal with nulls being returned by your SQL, perhaps a field where null means the user hasn't entered anything, and a zero means the user did enter a zero. In those case, you can test for null in your c#, like
if ( itot == null )
{
// do appropriate null stuff
itot = 0;
}
else
{
// do non null stuff
// nothing needed in this case
}