MySQL update

Hi, I have an update statement that tries to update a number of values - these values have unique value constraint so when I do the update it will either succeed or fail. If it fails how can I determine where the update failed. i.e. pinpoint the value that was not unique? At the moment I am doing ... $sql = mysql_query("UPDATE dis_user SET first_name = '$firstName', last_name = '$lastName', email_address = '$emailAddress' WHERE user_id = '$userId'"); if(!$sql) { echo "Data not inserted due error"; } How can I improve on the above to give specific error information. Thanks Matt