problem with subselect: NOT IN

Hi.

here's my scenario: table employee has emp_id and employee
details. table salesorder has emp_id and sales details.

The following works fine: (get all employees who have sold
something)

SELECT emp_id FROM employee WHERE emp_id IN (SELECT emp_id FROM
salesorder);

However, getting employees who have NOT sold something always
returns zero rows:

SELECT emp_id FROM employee WHERE emp_id NOT IN (SELECT emp_id
FROM workorder);

Has anyone encountered this before? I know the second query
should return something because the data is in the table.

thanks!

-Kevin

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