If you looking for logical correctness & practicality of using NULLs in
relational databases, then I would recommend you to go through Date's
Relational Database Writing 89-91 & 91-94 for an excellent treatment on the
issue. It is a very crucial & practical yet widely misunderstood concept and
usage of NULLs have been ingrained to our thought process by SQL databases.
Though Simon has suggested Joe Celko's book, I am a bit skeptical of the
suggestions in his book about using them, being biased based on SQL's flawed
approach to n-VL.

However, if you are just concerned with implementing a SQL table with or
without NULLs, with little concern over precise modeling practices, simply
read though the vendor manual. Nulls, irrespective of how often
characterized as markers for unknown/inapplicable values, are treated like a
"value" in most vendor DBMSs.

--
-- Anith
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