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However, care must be taken when trying to use NULL values in a
C<WHERE> clause.  Consider:

  SELECT fullname FROM people WHERE age = ?

Binding an C<undef> (NULL) to the placeholder will I<not> select rows
which have a NULL C<age>!  At least for database engines that
conform to the SQL standard.  Refer to the SQL manual for your database
engine or any SQL book for the reasons for this.  To explicitly select
NULLs you have to say "C<WHERE age IS NULL>".

A common issue is to have a code fragment handle a value that could be
either C<defined> or C<undef> (non-NULL or NULL) at runtime.
A simple technique is to prepare the appropriate statement as needed,
and substitute the placeholder for non-NULL cases:

  $sql_clause = defined $age? "age = ?" : "age IS NULL";
  $sth = $dbh->prepare(qq{
    SELECT fullname FROM people WHERE $sql_clause
  });

DBI.pm  view on Meta::CPAN

L<http://www.perlmonks.org/index.pl?node_id=288022>

Note: There is a bug in perl 5.8.2 when configured with threads
and debugging enabled (bug #24463) which causes a DBI test to fail.

=head2 Signal Handling and Canceling Operations

[The following only applies to systems with unix-like signal handling.
I'd welcome additions for other systems, especially Windows.]

The first thing to say is that signal handling in Perl versions less
than 5.8 is I<not> safe. There is always a small risk of Perl
crashing and/or core dumping when, or after, handling a signal
because the signal could arrive and be handled while internal data
structures are being changed. If the signal handling code
used those same internal data structures it could cause all manner
of subtle and not-so-subtle problems.  The risk was reduced with
5.4.4 but was still present in all perls up through 5.8.0.

Beginning in perl 5.8.0 perl implements 'safe' signal handling if
your system has the POSIX sigaction() routine. Now when a signal