Re: Does Codd's view of a relational database differ from that ofDate & Darwin? [M.Gittens]

From: Ged Byrne <ged.byrne_at_gmail.com>
Date: 8 Jun 2005 05:39:49 -0700
Message-ID: <1118234389.626550.232100_at_g43g2000cwa.googlegroups.com>

In TTM the empty set is modelled by TABLE_DEE and TABLE_DUM.

Empty sets are different from NULLs because an empty set tells you something.

If I ask for all children of John and the result is an empty set, then I know that John has no children.

NULL simply means UNKNOWN.

If the result to the children of John query is NULL, this means that I do not know how many children John has.

If anything, NULL probably relates to the universal set, since its presences means that anything is possible. Received on Wed Jun 08 2005 - 14:39:49 CEST

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