JOIN mysql help

[rukshilag]

//only completed successfully
if($cs =='yes') {
       $sql = "select * from past_participant where completion LIKE '%$cs%'";
       $result = mysql_query($sql);
       $num_rows= mysql_num_rows($result);
} else if($cs =='no') {
       $sql = "select * from course_participant FULL JOIN past_participant ON completion LIKE '%$cs%'";
       $result = mysql_query($sql);
      //$num_rows= mysql_num_rows($result);
}else if($cs =='current') {
       $sql = "select * from course_participant WHERE completion LIKE '%$cs%'";
       $result = mysql_query($sql);
       $num_rows= mysql_num_rows($result);
}

the above join doesnt seem to work in my code ... what is wrong??
it just needs to show all the "no" records in both tables.