Thanks to all of you... 
Issue has been resolved
function tgwhiteboard_getteacherdetail() {
global $DB;
$sql = "SELECT DISTINCT u.id, CONCAT(u.firstname, ' ', u.lastname) AS username FROM {user} u ";
$sql .= "JOIN {role_assignments} ra ON ra.userid = u.id ";
$sql .= "WHERE ra.roleid in (1,2,3,4)";
$teacherlist = $DB->get_records_sql($sql);
return $teacherlist;
}
id was compulsory 