Bob the Mushroom (4/13/2010)
Speaking as someone who opted for NULL instead of UNKNOWN, that explanation was terrible. Why doesn't NULL mean NULL, and what did empty strings have to do with anything? Was the question asking what NULL means in the context of a string variable?
It could have possibly have been phrased slightly better ("The fixed value NULL", or something like that), but it is correct. NULL does not equal NULL. Try comparing two NULL values in SQL - they will never be equal. A NULL field is actually unknown, so anything involving a NULL has an unknown value. Endless hours of fun for the unwary.
Have a look at 'about NULL values' in BOL.
BrainDonor.