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my @column_names = ("MO_resp","MT_resp","AO_resp");
my $sql = "select ".join ",", map { "sum($_)" } @column_names;
...
my $hour_sth = $hour_db->prepare( $sql );

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