Bob and Matthew, (01)
There's an excellent reason why nobody mentions NULL in SQL:
it's an embarrassment, which every expert wishes would go away. (02)
BN
> I am sure there is a really good reason why none of the experts
> on this thread have mentioned the "NULL' value in RDBMS practice,
> but for the life of me I cannot guess it. (03)
MW
> That is precisely the problem with NULL. You don't know if it means not
> known, or does not exist. In principle it could mean either in any
> situation. (04)
Yes. There is no way to define any rational semantics for NULL, and
many people have been trying to get rid of it (deprecate it) for years.
But once something goes into a standard and people start to use it,
it's extremely difficult to get rid of it. (05)
If you need a value to represent something that is known to exist
but has not been identified, it's better to choose some name
that does not normally occur in the domain. (06)
Many businesses would use 99999 for an unknown ZIP code. That caused
huge volumes of mail to be delivered to Alaska before it got sorted out
and sent back to the other 49 states. With more computerization, the
USPS now avoids shipping the physical mail to Alaska. (07)
John (08)
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