MySQL Advanced Query

Just a quick question... I am building a site where users log in and upload various listings in a table called 'listings_editor'. For each listing record, I require them to enter a unique listing id number for the listing. What I would like to do is create another table for users to submit additional data about any of their listings, and I want them to be able to select from a list which one of their listings they are submitting additional info for. I am using the MySQL query feature for this, and so far, I have:

SELECT listing_number
FROM `<?php echo $TABLE_PREFIX ?>listings_editor`

This gives me a list of all listing numbers in the 'listings_editor' table. What I want to do is add a filter so that only the listing numbers for the current user are displayed. My thought is to do the following:

SELECT mls_number
FROM `<?php echo $TABLE_PREFIX ?>ad_copy_editor`
WHERE createdByUserNum = $userNum

The problem is, I don't know how the user number is stored and accessed across the CMS system for reference. Any help would be appreciated!

Thanks!

PS. All the users uploading listings are given "Author" access to 'listings_editor'