-- Randal L. Schwartz, Perl hacker

But just to be absolutely clear about it, even though '' eq undef, defined('') == 1 while defined(undef) == 0. Which is why that function is there, I suppose.

Philosophy can be made out of anything. Or less -- Jerry A. Fodor

Heh, how long this thread has gone on ... so, to distinguish null from empty string in CSV, you could to

my $csv = qq{foo,,bar,''};
[download]

And write your code so that the second item in that list is NULL, while the fourth is a blank string. Unfortunately I don't know if there's a standard for that or not. I suppose I should try both in DBD::CSV and see what pops up. (update down the road)

as promised here's some code ... I think the upshot is that DBD::CSV doesn't distinguish between blank strings and undef values, but I can't tell because the code isn't working ATM. posted for educational purposes only.

#!/usr/bin/perl -w

use DBI;
use strict;

my $db = DBI->connect("DBI:CSV:f_dir=/home/arturo/testing")
        or die "Cannot connect: $DBI::errstr\n";

=pod

# used to create the table
my $sth = $db->prepare("CREATE TABLE foo (id  INTEGER,
                                          firstname VARCHAR(32),
                                          lastname      VARCHAR(32) )"
+);

$sth->execute;

$sth = $db->prepare("INSERT INTO foo (id, firstname, lastname) VALUES 
+(?,?,?)");
$sth->execute(1, 'Charo', '');
$sth->execute(2, 'Wynonna', undef);
$sth->execute(3, undef, 'Cher');
$sth->execute(4, '', "Sting");

# note: Charo has a blank last name, Wynonna has an undef one
# Cher has undef first name, Sting has blank first name

=cut

my $sth = $db->prepare("SELECT id, firstname, lastname FROM foo") 
    or die "Cannot prepare: $DBI::errstr\n";

# next line currently causes an error on my system 
# (fresh installs of all the modules on which DBD::CSV depends)

# UPDATE the answer, of course, is to execute it first 
# /me LARTs himself

$sth->execute();

while ( my ($id, $first, $last) = $sth->fetchrow_array() ) {
        print "First name '$first' is " .
                ($first eq '' && defined $first) ? "blank" : "undefine
+d";
        print "Second name '$last' is " .
                ($last eq '' && defined $last ) ? "blank" : "undefined
+";
}

$db->disconnect;
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As I mentioned, looking at the file that got created, I don't see a difference between blank and undef, EXCEPT in the first two cases: the "Charo" line has a trailing comma, the "Wynonna" line does not. And thanks to busunsl for pointing out I forgot to execute my statement (sigh, it's not the best of days for me when it comes to details).

Philosophy can be made out of anything. Or less -- Jerry A. Fodor