Wordpress SQL query - returning 'true' 'false' or 'null'

Question

I am working on coding a PHP form for my website that requires the user to enter a code before the form becomes active. I have a field called "used" that I set up as a Boolean/TinyInt which I thought would return 0, 1 or 'null' but it doesn't seem to be working like that.

Currently I have 2 entries in this database, one of them has a 1 in the 'used' field and the other has nothing(null) but for some reason, both of those queries return "1."

All of the codes will be entered in the database with 'null' assigned to the "used" field by default. When they enter their code, it first checks to see if that code exists in the database, if it does, then it checks the 'used' field to see if someone has already registered using that code.

I am quite new to WP queries, so could anyone let me know what the best way to accomplish this would be? Here is my code so far:

$enteredCode = $_GET['code'];

global $wpdb;

$codeQuery = $wpdb->query("SELECT used FROM rpp_codes WHERE code='".$enteredCode."'");
$exists = $wpdb->get_results("SELECT used FROM rpp_codes WHERE code='".$enteredCode."'");

if (count ($exists) < 1) {
    echo "invalid";
} else {
    if ($codeQuery == 1) {
        echo "true";
    } else {
        echo "false";
    }
}

Answer 1

$wpdb->get_results() returns an index array of row objects, but it looks like you're expecting $exists to contain the actual value of the used column. you probably want $wpdb->get_var(): http://codex.wordpress.org/Class_Reference/wpdb#SELECT_a_Variable