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Home -> Community -> Usenet -> comp.databases.theory -> Re: So what's null then if it's not nothing?
vc schrieb:
> Alexandr Savinov wrote:
>
>>Jon Heggland schrieb: >> >>>In article <1134926774.703015.67240_at_z14g2000cwz.googlegroups.com>, >>>boston103_at_hotmail.com says... >>> >>> >>>>Sure, a one-argument predicate like 'is_null', or 'is_odd', talks >>>>about a property/attribute and can be called an 'adjective'. But the >>>>'is_null' predicate cannot help one answer the question "what is '+' in >>>>'2 + NULL' ?". So, what is '+' in the expression and how does one >>>>interpret such expression ? >>> >>> >>>"2 + NULL" isn't a valid expression. (Well, it is in SQL(?), but I don't >>>think it should be, and I won't try to justify it.) >> >>2 + NULL = 2
>>Just remove NULL because it is a flag to denote a non-existing thing >>(which exists in other dimensions but does not exist here in the current >>dimension). So there are neither formal nor informal problems with this >>expression if we provide correct interpretation of NULL. Notice that >>NULL here (and always) in not number 0. In the expression 2+NULL we do >>not apply the operation at all,
This question is somewhat analogous to "how do we reconcile that thing does not exist and thing is being processed". The answer is that things can exist (and be absent) along many dimensions.
More concrete answer is that 2 + NULL should not appear if we use a correct theory because 2 is an element of a domain (a thing, normally plotted horisontally) while NULL is an element of a semantic/logical evaluator like true or false (plotted vertically). We can introduce operations between entities or between logical values (which evaluate entities) but not between them. Yet, currently such "expressions" as 2+NULL are possible, however, if we can easily avoid any problems if NULL is interpreted as SKIP. Effectively, this means that operations (with things) cannot be applied to NULL -- precisely what we need in a good theory:
2 + NULL = 2 + SKIP = 2 (+ IGNORE) = 2 I agree that formally 2 + NULL is a strange thing (illegal from algebraic point of view) but since it happens we can find an easy explantion for it and solution for the problem. In some sense it can be compared with infinity in mathematics which is a formally illegal element but there are some tricks which allow us to view it as a normal element. I beleive that NULL is a representative of NOTHING as an oppsite of INFINITY in mathematics. In other words, we have infinity and we need to have also nothing, and NULL is a good example for defining ist formal properties.
>>while in 2+0 we do apply the sum operation. >> >>-- >>http://conceptoriented.com
-- http://conceptoriented.comReceived on Mon Dec 19 2005 - 06:06:16 CST
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