sub make_booking_SQL {
    my ($BookingID, $ref_booking, $ref_event, $ref_course) = @_;

    return sub {
        $sth = $dbh->prepare("SELECT * FROM booking WHERE BookingID = $BookingID") or die $dbh->errstr;
        $sth->execute();
        $ref_booking = $sth->fetchrow_hashref;

       $sth = $dbh->prepare("SELECT * FROM event WHERE EventID = $ref_booking->{'EventID'}") or die $dbh->errstr;
       $sth->execute();
       $ref_event = $sth->fetchrow_hashref;

       $sth = $dbh->prepare("SELECT * FROM course WHERE CourseID = $ref_booking->{'CourseID'}") or die $dbh->errstr;
       $sth->execute();
       $ref_course = $sth->fetchrow_hashref;
    };
}