for my $cd (@cds) {
    print  $cd->name, $/;
    printf " - %d: %s\n", $_->num, $_->song->name
        for sort { $a->num <=> $b->num } $cd->tracks;
}

##</code><code>##

my $select = sprintf "select %s from $table where %s",
    join(","     => @cols),
    join(" and " => map { "$_=?" } keys %constraints);

my $sth = $dbh->prepare($select);
   $sth->execute( values %constraints );

#########

my $insert = sprintf "insert into $table (%s) values (%s)",
    join("," => keys %data),
    join("," => ('?') x scalar keys %data);

my $sth = $dbh->prepare($insert);
   $sth->execute( values %data );

##########

my $update = sprintf "update $table set %s where %s",
    join(","     => map { "$_=?" } keys %data),
    join(" and " => map { "$_=?" } keys %constraints);

my $sth = $dbh->prepare($update);
   $sth->execute( values %data, values %constraints );