How to compare user entered element value with DB content?

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What steps will reproduce the problem?
1. create table "user" containing "user_name" in a mySQL DB
2. fill in a user-name
3. add element "user_name" to a form
4. display form, fill in a user name
5. submit 

What is the expected output? What do you see instead?
TRUE / FALSE
and, if FALSE 
An error msg telling me that the user name entered is already in use

What version of the product are you using? On what operating system?
the newest one

Please provide any additional information below.

Question:
How can I check wheter a user name is already used (stored in the db) before 
and after submit?

Any ideas?
thanx :-))

Original issue reported on code.google.com by i...@oeko-travel.org on 25 Jun 2014 at 1:04