There are two ways.
First, you can use a subquery to find all the users in the first table NOT IN the second, as:
SELECT User FROM TableA WHERE User NOT IN (SELECT User FROM TableB)
or, being trickier, you can use an OUTER JOIN of the two tables and select the User from the TableB columns of the result where the User is NULL:
LEFT OUTER JOIN TableB ON TableA.User = TableB.User
WHERE TableB.User IS NULL
admittedly, the second query is harder to understand.
It's not clear which is more efficient. Indeed, they may result in the same execution plan. Or not.
Custom Apps, Inc.