I don't understand - in your code you've hardcoded to NULL, better to omit columns if you want to always have the values empty in your code like this:

INSERT INTO nodes ( column1, column3) values ( ?, ?)
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I thought your question was about what happens when you want to execute a query and some of your variables are empty. This is what I get when working with DBI, DBD::mysql.

mysql> show fields from emptycols;
+-------+-------------+------+-----+---------+-------+
| Field | Type        | Null | Key | Default | Extra |
+-------+-------------+------+-----+---------+-------+
| a     | varchar(12) |      |     |         |       |
| b     | varchar(5)  | YES  |     | NULL    |       |
| c     | int(11)     | YES  |     | NULL    |       |
+-------+-------------+------+-----+---------+-------+

my $dbh = DBI->connect('dbi:mysql:test',$ENV{'USER'});

my $sth = $dbh->prepare("INSERT INTO emptycols VALUES (?,?,?)");

$sth->execute("apple", "red",17);
$sth->execute("apple", undef,10);
$sth->execute("orange", "ora",undef);

mysql> select * from emptycols;
+--------+------+------+
| a      | b    | c    |
+--------+------+------+
| apple  | red  |   17 |
| apple  | NULL |   10 |
| orange | ora  | NULL |
+--------+------+------+
3 rows in set (0.00 sec)
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