How to return records that do not match the list of UID's from a previous query?

So this initially (and probably still is) seemed like a simple task.

I am getting a full list of UIDs from a SC based on an id on the page.

Then I’m running a query to check if any of those UIDs appear in another table, and return those that do.

What I want to do is return another query of the UIDs from the full list that DO NOT appear in the second table.

Perhaps it’s just getting late here. I thought the inverse of the second query and in the join use a NOT EQUAL instead of EQUAL - but this just doesn’t work.

Any suggestions appreciated.