sql question (hopefully)

will try to keep this as short as possible. =)

1. "select id from a_table where id not in (2,3,4)" works fine

2. "select id from a_table where id not in (select id from b_table)" works
fine if the output of the "select id from b_table" looks like:
id
----
 2
 3
 4

problem is "select id from b_table" in #2 statement has the following
output:
id
----
2,3,4

how do i do execute the query in a single sql statement?

thanks in advance.

mel