Can SQL Assign Zero When NULL Is Returned From A Sub-query?

Hi Folx,
I am new to
SQL Server. I am rolling summed values from table profile to two columns in table txn. When the value for one (not necessarily both...) column is null, can that value be translated to zero? Or must I write a procedure or use a cursor?
Versions:
Microsoft SQL Server Integration Services Designer
Version 9.00.1399.00

Microsoft SQL Server Management Studio
9.00.1399.00

For example, when version_count for a particular transaction_id is null in the profile table but profile_count has summable integers, how do I update the version_count to zero in the same pass that I update the profile_count?

update txn
set
version_count =
(
select
sum(a.version_count)
from
profile a
where
a.transaction_id=txn.transaction_id
and
a.extraction_date=txn.extraction_date
and
txn.version_count is null
),
profile_count =
(
select
count(*)
from
profile a
where
a.transaction_id=txn.transaction_id
and
a.extraction_date=txn.extraction_date
and
txn.profile_count is null
)