Comparing data in two tables to find missing data

[new7media]

I have a small web site where people can register and create an account, and then add links to our site.

I have two tables set up. One holds registration data -- user_id, username, password, date registered. The other holds link data -- link_id, user_id (to track which registrant added which link), and URL.

Some people have created an account, but have never posted a link. I'd like to figure out who those folks are but I'm not quite sure about how to do that. If I did this manually I would get a list of user id's from the registration table and then look at the link table and check off user id's in that table. The unchecked user id's are my target group.

I'm at a loss as to how to do this in a MySQL query. Or if I need to use PHP to process this data set. Or what....? Any clue, any help is really appreciated. Thanks!

[tsinka]

Hi,

you can get the ids with a left join:

SELECT r.user_id FROM registration r LEFT JOIN linkdata l ON r.user_id=l.user_id WHERE l.user_id IS NULL

Thomas

[new7media]

WOW! LEFT JOIN! WHERE x IS NULL! YES! Thanks so much!