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

David Cressey wrote:
> <michael_at_preece.net> wrote in message
> news:1133490255.501081.275050_at_g44g2000cwa.googlegroups.com...
>
> > Sorry vc - was it Jon that made that point? Can Jon, or perhaps someone
> > else, explain how an empty string differs from a NULL string? I don't
> > want to know about a place-holder for a missing string - I mean a
> > string that is known to be empty.
>
>
> They differ in two ways: in representation and in interpretation.
> The difference in representation is an implementation detail. The
> difference in interpretation might need some explanation.
>
> The number ZERO is the identity over the operation of addition. This means
> that ZERO has the nice property that:
>
> X + 0 == 0 + X == X for every integer X
>
> Among mathematicians, some wag has said, "if the number ZERO did not exist,
> it would be necessary to invent it."
>
> The EMPTY string (the VARCHAR string of length zero) is the identity over
> the operation of concatenation, a fundamental operation on strings. EMPTY
> can be represented in SQL by a couple of single quotes with nothing in
> between them. For
> purposes of these postings, I prefer to use the word EMPTY. EMPTY has the
> nice property that
>
> A || EMPTY == EMPTY || A == A for every string A.
>
> So EMPTY plays the same role with respect to concatenation that ZERO plays
> with regard to addition. People who are doing simple things like mailing
> lists often don't need top operate on strings in a mathematical sense. But
> there are some operations on strings that do require this.
>
> Now the history of ideas is such that all flavors of nothingness begin as an
> abstraction, indicating the non existence of something real, and then take
> on a reality of their own.
>
> For centuries, the number represented by the single cipher ZERO was used to
> represent, "there isn't a number here".
> Then ZERO was added to the set of integers, and it no longer meant what it
> used to mean. It was a number, like any other number.
>
> The cipher ZERO went through a similar evolution. Originally, in decimal
> place value notation, a little dot, looks kinda like a decimal point, was
> used to indicate "there isn't a cipher here". So 1..3 meant one thousand
> and three, while 13 meant thriteen. Then the Cipher ZERO was added to the
> set [1,2,3,4,5,6,7,8,9] and it grew into the oh shaped cipher we know today,
> and the set now looks like this: [1,2,3,4,5,6,7,8,9,0].
>
>
>
> A similar evolution happened to the ASCII NUL (char(0)). In the days of
> paper tape and teletypes, NUL was used to indicate "tape feed". Tape feed
> was a series of sprocket holes, with no data punched in the eight data
> tracks. It was understood as "there isn't a character here". At the time
> the C language was developed, they adopted the convention that, since NUL
> was unnecessary, it could be used as a sentinel. indicating the end of a
> variable length string.
>
> But what if the very first character is a NUL? Now the empty string is
> born.... Yet another flavor of nothingness.
>
> Now, you wouldn't use NUL to represent ZERO (either the number or the
> cipher). You wouldn't use NUL to represent SPACE. And, if you want to
> represent the empty string as a string, you wouldn't use it to represent
> the absence of a string.
> The Oracle engineers unfortunately did just this, and it's a mistake. The
> mistake is not apparent when you do mailing lists, but if you use strings
> for linguistic analysis, it comes back to bite you.
>
> There are two cases of nothingness that I haven't dealt with yet: the Lisp
> "nil", and the empty set "[]". I'll skip the lisp nil, because it's beside
> the point here. The empty set is NOT the absence of a set. It's one of the
> possible sets. So if you have a container that might contain a set, or
> might not contain a set, you need two representations:
>
> one for "a set is present and it's empty" and the other for "no set is
> present". If you use the same representation for both cases you can't tell
> them apart. This distinction and its necessity ought to be apparent to you,
> even if the EMPTY and NULL distinction is not.
>
> This is about the fifth time I've tried to explain this. I don't know how
> to make it any clearr.

That's a fantastic post, and one of the best on this thread. I may well utilise some of its contents for teaching with, if that is okay with the author. Received on Fri Dec 02 2005 - 14:02:23 CET