3vl 2vl and NULL

I've stayed away from the 3vl versus 2vl discussion over in the monster NULL
thread. Mainly, I'm not sure exactly what Codd and Date have to say on the
subject. I think that both of them are smarter than I am, and that they
don't agree on the subject, and that they agree that they disagree. That
pretty much leaves it up to us, doesn't it?

Here's what I think: There are two Boolean constants: FALSE and TRUE.
A boolean variable with a value must have one of these two values. I think
that's the 2vl position, although I'm not sure.

Any variable, including a Boolean variable, might have no value assigned,
in which case it's missing data. Systematic treatment of missing data
requires some kind of treatment for this case.

Now, since the term NULL has produced so much controversy, I'm going to use
an alternate notation in this post. I'm going to use the marker "(no
comment)" to indicate a missing value. Now let's go through some cases,
in Q and A format.

Q: is 4 greater than 3?
A: TRUE

Q: is 4 equal to 4?
A: TRUE

Q: is 4 equal to 3?
A: FALSE

Q: is 4 greater than (no comment)?
A: (no comment)

Q: is (no comment) equal to (no comment)?
A: (no comment)

Q: How much is 4 plus (no comment)?
A: (no comment)

Now, (no comment) doesn't mean "either TRUE of FALSE, but I don't know
which". It doesn't mean "neither TRUE nor FALSE but some other boolean
constant you never defined". It doesn't mean "I know the answer, but I'm
not telling you".
All it means is: (no comment). Draw whatever inferences you like.

Now let's try some data:

Q: select spouse_first_name from employees where employee_id = 12345;
A: (no comment)

It doesn't mean "This employee has no spouse". It doesn't mean "this
employee has a spouse, but one with no first name". It doesn't mean "This
guy's spouse's first name is unknown to us".

All it means is that, when the author of the row filled in the values, all
the author said about spouse_first_name was: (no comment).

>From here you can draw whaterver inferences you like. There may be
inferences you can draw by agreement between the writer and the reader. In
this sense, NULL can be defined in whatever way is most useful, as
Marshall Spight might say. But in terms of what has been SAID , regardless
of what it might mean, all that's been said is NULL or, in my phrasing (no
comment).

I think this boils down to Codd's position, but I'm not sure.





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Relevant Pages

  • Re: 3vl 2vl and NULL
    ... Systematic treatment of missing data ... Draw whatever inferences you like. ... it might return an empty set ...
    (comp.databases.theory)
  • Re: Frege: Reasons nearest kin
    ... "Reason is the power to think rationally and logically and to draw ... inferences Intuition is perception or comprehension, ...
    (sci.logic)
  • Re: Frege: Reasons nearest kin
    ... "Reason is the power to think rationally and logically and to draw ... inferences Intuition is perception or comprehension, ...
    (sci.logic)
  • Re: Sampras on carpet
    ... > Use some logic and draw your own inferences. ... Or explain "obvious but unstated multiple win", because my English no good. ...
    (rec.sport.tennis)
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