Doing Math Across Table Columns, Dealing With NULL

Christine the Pharmacist writes "Please pardon as I am not horribly advanced with SQL...

Using SQL Server 2000, Windows 2000, Service pack 4

I'm trying to figure out marketshares for "preferred" products. I have a table put together by hospital and month of medication use with the totals of each drug dispensed in columns as well.
(example table below)


Hospital Month Drug A Drug B Drug C Drug D
A 7 5 10 58 73
B 7 NULL 26 98 43
etc.


** Drug C and Drug D are the preferred drugs

I'm getting correct preferred % in all cases except when a value is NULL (result is NULL). I'm using an aggregate (sum), which should ignore NULLs, but it ignores NULLs within a column, and not ignoring across columns. I've tried setting concat_null_yields_null off and I'm still getting NULL (since I'm using the plus sign).

This is my admittedly confused select statement, suspect this is where my problem lies (problems with saying "sum", but using "+" as well?):

select a.Hospital,a.month,convert(float,sum(c.Drug_C+d.Drug_D))/convert(float,sum(a.Drug_A+b.Drug_B+
c.Drug_C+d.Drug_D)) as preferred_share

Thanks for any advice!"