subquery vs set operator

I was having trouble getting a set of values using a subquery, but was sucessful with using a set operator. Although the problem is solved, I am curious as to why the first solution did not work. I have two tables, A and B. both use objid as a PK, and both have a column for employee_no. There are rows in table A that do not have a corresponding row in B.

When I tried this:
Select employee_no from B
where employee_no not in
(select employee_no from A);

This returns no rows.
However, when rewritten as:

Select employee_no from table A
Minus
Select employee_no from table B;

I get the correct values.
Can anyone help me figure out why the first query is not working? Is there a logic error here? Three of us in my company who are fairly experienced with SQL are stumped by this one.

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Before you buy. Received on Fri Sep 08 2000 - 12:50:17 CDT