SQL & PL/SQL :: Update Statement With Join Only Update Matching Rows

This is my query:

UPDATE t_tt_hours a
SET a.sak_request = (
SELECT b.sak_request
FROM t_requests b, co c

[Code]...

The problem I am having is that it is updating all rows even when it is pulling back a null value for b.sak_request. I've tried adding b.sak_request is not null to the select statement like this:

UPDATE t_tt_hours a
SET a.sak_request = (
SELECT b.sak_request
FROM t_requests b, co c
WHERE b.nam_eds_tracking_id = c.id_dir_track_eds

[Code]...

but it doesn't seem to make a difference. The reason I need to do this is that the difference between where it matches with a valid (non-null) value is 396 rows vs. 12,484 rows which is too time consuming to run on my page.