Simple While loop database search table

first of all: Thanks for all the great help I already received through finding answers to questions others posted.

I have a small and easy question for you:

I'm trying to randomly generate a number, but if it exists in the database table, it should keep generating a new numbers until it finds a unique number.

Help would be much appreciated!

    $klantnr = rand(1,9);

    $kn = mysql_num_rows(mysql_query('select username from users where id="'.$klantnr.'"'));

    while($kn!=0){
       $klantnr = rand(1,9);
      }                                   
echo $klantnr;