So, I'm coding a registration form, and I want to query the database to make sure that the username the user wants to register with is unique, and not already in use. How can I do this?

Thanks in advance.

    Make the username column UNIQUE. If there is an error on insert, check the error to see if it is because of a UNIQUE constraint violation.

      use laserlight's suggestion to force uniqueness in the database

      before you register someone, check the username isn't already taken with a query something like this (w/ your own table and var names of course):

      $sql = "select * from user_table where username like '" . 
            mysql_real_escape_string($username) . "'";
$res =mysql_query($sql);
if(mysql_num_rows($res) > 0) {
  // user name already taken
}

      if you define the username column in the table w/ unique index constraint, it will help look up efficiency too:

      constraint unique index(username)

        before you register someone, check the username isn't already taken with a query something like this (w/ your own table and var names of course):

        According to those around here more skilled at databases than me, that leave a small window for a race condition:
        1. The script checks that the username is not taken.
        2. Another user registers the username.
        3. The script attempts to insert the username.

        With a unique constraint, step #3 now results in a failure that you still must handle. Consequently, you might as well just insert the username and handle the failure, if any.

          In that case, would you check the mysql errno to see if it was a duplicate key error?

            In that case, would you check the mysql errno to see if it was a duplicate key error?

            Yes.

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