By saying "But none of those are a one-to-one match with 'NULL'", you both deny a simple answer and leave some information off the table that would be helpful to those coming at this from an SQL perspective.

The simple answer, which is probably "right" for much of the audience is to explain undef and how you test for it. For someone fetching a row of fields out of a database, where some of the field can be NULL, this answer is good enough.

Where this answer goes awry is in SQL comparison semantics. In SQL, you test for the presense of NULL using an "IS NULL" clause, because NULL is never equal to NULL. A frequent source of SQL newbie errors is preparing   SELECT a FROM t WHERE b = ? and then binding NULL (or undef) to the placeholder. No go. One needs to write   SELECT a FROM t WHERE b IS NULL Granted, this is a SQL problem and not a Perl problem, but it's lurking there waiting to bite the uninformed. In my opinion, it's worth mentioning whenever any asks about how to do NULL in Perl.

In reply to Re: How do I test for a NULL string? by dws
in thread How do I test for a NULL string? by merlyn

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