On Submit Validate info with Database

Hallo

I hope some one can help me i have a webpage that post details enter on the
page into a Database. Which works fine but what i want is before it submits
the info to the database i want to verify if the User-ID enter in the form is
in my user database if yes then submit else say that the User-ID is wrong.

Is this possible if yes how.

Regards
Markus