PHP registration script: verifying and telling the user what went wrong

I'm building a registration script and I want to insert the user's input (username, mail) ONLY if it's not already in the database. What I usually do in such cases is a request to see if something's already there, something like: "SELECT * FROM things WHERE thing_name = '$treated_user_input'".

I have two fields that need to be unique this time though. Is there a way to do only one SELECT request and still be able to tell the user exactly what field went wrong?开发者_如何学Python Or do I have to do one request per unique field?

Thanks for your answers.

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Look at SQL distinct for 2 fields in a database for selecting two distinct fields. At that point, if you have at least one result, you can use PHP to determine which is the offending field.