Re: So what's null then if it's not nothing?

> "mAsterdam" <mAsterdam_at_vrijdag.org> wrote in message
> news:4380531c$0$11064$e4fe514c_at_news.xs4all.nl...

> Yes.  Definition (3) above should be the lead definition, and should
> explicitly state what the SQL definition is, namely a marker
> indicating the absence of a value.
>
> Defnitions (1) and (2) above should clearly be distinguished as
> INFERENCES (possibly valid) drawn by the reader, rather than
> INDICATIONS conveyed by the NULL itself.  In other words,
> definitions (1) and (2) attempt to communicate WHY a value is not
> present, which goes beyond the purpose for which NULL was invented.

I think I'd be inclined to start with something like (3), with some indication of both the variations on its handling and of the multiple interpretations that may be made.

Thus...

• NULL is a special marker returned by SQL indicating the absence of a value.
• Different SQL implementations handle this marker differently. In some systems, it is treated as being equal to some sort of "empty/blank/zero/FALSE" value. In others, it is handled as a special kind of value that can never be equal to any value.
• There are numerous inferences that may possibly be taken from receipt of a NULL marker, including:
  • The result exists but was not recorded in the database

    Someone may fill in a survey and decline to indicate their gender.     They presumably have one; it just was not recorded.  

• The result had no applicable value

    In the case of the gender field, above, there do exist truly     unusual cases where gender is not well defined, such as where an     individual has the organs associated with *both* genders.

    Alternatively, severe injury could effectively eliminate gender.

    Consider a sales representative who works solely on commission. A     query that searches for annual salaries should find none for this     individual, as they receive no "salary" per se.

• The result is impossible (e.g. - inconsistent with premises)

    "When did you marry your spouse?" has no meaning for someone who     has never been married and has no spouse.

    Asking for the salary of someone who is not employed should find     "nothing," but it is not precisely the same kind of nothing as the     "salary of the 100% commissioned sales rep."

There may be other interpretations of NULL; these are just the most obvious/common...

-- 
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