>NULL really means that the value is unknown. It's very different from an empty value. You might need to add OR ISNULL(c) to your WHERE clause.
>

One definition I really like for NULL, is "Doesn't make sense". Say, for instance, in an "animals" table, the field "How_many_eggs". It wouldn't be zero for the Elephant as it doesn't make sense.

The fact is, that lots of people leave the the nulls on, meaning "allow blank".
In the few cases I needed nulls, I often wondered if that record belonged to that table or if I was just being too generic.

I know this slips off a bit on the topic. Just a thought on design.