Mark Kirkwood <markir@paradise.net.nz> writes:
> The relational model was designed using a 3 valued logic - true, false,null.
> All relational database implementations will inflict this on you :-)
Not sure that it's fair to characterize this as a property of the
relational model. It is a property of the SQL standard. There are
many purists who say that SQL is not really relational at all (Chris
Date being one of the more prominent ones, IIRC), but in any case,
SQL drew three-valued logic from other sources than the relational
model of databases.
However, it is true that all spec-conforming implementations of SQL
will inflict this on you. MS SQL Server, for one, has apparently been
non-compliant on this point in the past, and I'm not too sure about
Oracle.
regards, tom lane