Check if record exist then submit

Hi @ all
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I am trying to create a joomla registration form on my site. But i want only users that have their records in the database to be allowed to register. Such that when the user fills and submit the username, if it tallies with a username record in the database, then the user will register successfully, if not deny user registration.
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I have created the form, uploaded the database records with the required usernames; i have tried using the read data but its not working, cause i think am not sure how to configure the parameters properly.
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Your kind assistance is required.
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Regards.

Hi foluso123,
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Please turn on the Debugger or add a Debugger action to the On Submit event. That should show you the DB Query that is being generated and the resulting data.
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Bob