The SQL that will do what you want is:

SELECT n.id
  FROM nodes n, meta_data md
 WHERE md.name = "color"
   AND md.value = "green"
   AND n.id = md.node_id
 ORDER BY 1
[download]

To make this useful, do something like

my $sql = <<'__END__';
SELECT n.id
  FROM nodes n, meta_data md
 WHERE md.name = ?
   AND md.value = ?
   AND n.id = md.node_id
 ORDER BY 1
__END__

my $sth = $dbh->prepare_cached($sql) || die $!;
$sth->execute($attr, $value) || die $!;
# Use your favorite fetch*() method here
$sth->finish;
[download]

Placeholders are your friend.

------
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