Re: 3vl 2vl and NULL

In article <g4a1q158gmk0vnklaogelgfcpgsnkb027q@xxxxxxx>,
hugo@xxxxxxxxxxxxxxxxxxxxxx says...
>
> I object to the choice of words "It is unknown", because in fact Uncle
> Vernon's age IS known. Not to you and me, but it is known to many
> others.

I'd rather say that the important thing is that Uncle Vernon *has* an
age, even if nobody (including himself) knows it.

> >This sounds like an agreement on that "value is missing" is essentially
> >the same as "value is unknown". Or do you draw some separation between
> >"unknown in the database" and "unknown in the real world"?
>
> You've got it.
>
> In many of the discussions about NULL, people get distracted by the
> notion that NULL is suggested to mean "unknown".

I though that *was* the discussion. :)

> Then they find examples
> where there's other reasons to use NULL (inapplicable, e.g.), which
> leads to the discussion that there should be two, three, or whatever
> number of NULLs to denote the various reasons why data can be missing.

I thought it was commonly accepted that "inapplicable NULLs" are an
artifact of wrong database design. Inapplicable data are *not* missing.
Furthermore, to use SQL NULLs for "inapplicable" does not make sense
with the current rules; that is my point. It should be an error to try
to perform an operation on inapplicable data; it should not just result
in another NULL or in an UNKNOWN truth value.

> To avoid that misunderstanding as much as possible, I always make the
> distinction between the MEANING of NULL ("no value is here") and the
> RESULT of NULL (that the DB doesn't know which value to substitute for a
> variable or column name in an expression).

If this is the RESULT (with capital letters) of NULL, then it seems
obvious that it means "unknown", and not "inapplicable". If it could
mean "inapplicable", there wouldn't *be* any value to substitute.

> >So the fact type for the one tuple is not the same as for the other.
> >
> >It seems, though, that you use "fact type" in a manner unfamiliar to me.
> >In the descriptions of the RM and predicate logic I have seen, "fact
> >type" is synonymous with "predicate", and each relation has one (and one
> >only, though different (equivalent) formulations are of course
> >possible). (Though of course, NULLs have no counterpart in predicate
> >logic that I'm aware of.)
>
> My background is NIAM, a Dutch version of ORM. That's probably why you
> are unfamiliar with my use of the term "fact type". You're right that
> the terms "fact type" and "predicate" are almost synonymous.
>
> In ORM/NIAM, each predicate has it's own "table". As a result, there are
> no NULLS in ORM/NIAM. For the example above, there simply would be no
> fact at all for Uncle Vernon in the fact table for a person's age.

I'd say that this is how the RM works too. In my opinion, ORM/NIAM is a
graphical representation of the RM.

> When mapping ORM/NIAM to RM, a fixed set of rules dictates which fact
> types are combined. The resulting tables combine the data from the
> combined fact tables.

But you don't need to combine fact types. It's just as much RM (more, in
fact, since you avoid NULLs:) if you don't. Do the rules ever produce
NULLs that don't mean "unknown"?

> In the readings above, I assumed that the tuples (Aunt Marge, 47) and
> (Uncle Vernon, NULL) are the result of combining a unary fact type that
> enumerates the people in my family with a binary fact type that holds
> the age for members of my family.

Based on the reasonable assumption that all your family members have
ages, I guess. And in that case, I think it is more correct to interpret
the tuples as

- My family member Aunt Marge has an age of 47 years.
and
- There exists an age X so that
My family member Uncle Vernon has an age of X.

---i.e. that tuples with NULLs have the same interpretation as when the
NULLs are projected away.
--
Jon
.

Relevant Pages

  • Re: TSQL INNER JOINS and null fields
    ... NULL can be best thought of as "Unknown". ... Name Age Job ... Paul Programmer ... A better solution is to include the possiblity of NULLs in the query. ...
    (microsoft.public.sqlserver.programming)
  • Re: So whats null then if its not nothing?
    ... The IBM UniVerse programmers manual has this to say about the NULL ... Unknown Data: The Null Value ... Do not confuse the null value with the empty string. ... SORT EMPLOYEE FIRSTNAME SEX AGE ...
    (comp.databases.theory)
  • Re: 3vl 2vl and NULL
    ... >>Dot as everyone else, including Uncle Vernon in this 2VL scenario, has ... >>If the data are changed so that Aunt Marge also has a null set age, ... >>someone whose age (in this database) is less than or equal to Marge's ...
    (comp.databases.theory)
  • Re: 3vl 2vl and NULL
    ... >>>someone whose age (in this database) is less than or equal to Marge's ... >Uncle Henry's age is unknown ... the handling of missing data changes as well. ...
    (comp.databases.theory)
  • Re: So whats null then if its not nothing?
    ... But not different to something with the value '' (aka empty string). ... Both Null and Unknown are important issues to understand when using SQL. ... What can you tell me about Joyce's age? ... such as Age - since a blank cell clearly ...
    (comp.databases.theory)