my @column_names = ("MO_resp","MT_resp","AO_resp");
my $sql = "select ".join ",", map { "sum($_)" } @column_names;
$sql .= " from ? where Date = ? and Hour like ?";
my $hour_sth = $hour_db->prepare( $sql );

$hour_sth->execute($table_name, $db_date, "$hour:%") or die $DBI::errstr;