Re: cdt glossary 0.1.1 [NULL]
From: mountain man <hobbit_at_southern_seaweed.com.op>
Date: Wed, 14 Feb 2007 02:43:18 GMT
Message-ID: <aHuAh.824$4c6.772_at_news-server.bigpond.net.au>
>>> [NULL]
>>> Roughly: a special marker that can be put in a place
>>> inside a data structure where an actual value is expected.
>>> Precisely what that marker means varies and there are at
>>> least three possibilities that are sometimes assumed:
>>>
>>> (1) "Unknown value" This means that on the place of the marker
>>> there should actually be a value but this value is not known
>>> at the present time. For example, if a 'name' field in a tuple
>>> describing a person is 'null' then this person will have a
>>> name but we don't know it.
>>>
>>> (2) "Absent value" This means that the property that is
>>> described by the value in question is simply not defined.
>>> For example, if the 'shipping-date' field in a tuple
>>> describing an order is 'null' then the order was
>>> not shipped yet.
>>>
>>> (3) "Whatever SQL says it means" The exact meaning is hard to
>>> summarize briefly, but is a mixture of the previous two
>>> interpretations and involves a value with three truth-values
>>> ('true', 'false' and 'unknown').
>>>
>>>
>>> Common usage:
>>>
>>> - Confusion arises when people use terms like "null value",
>>> a paradox to some, a contradictio in terminis to others.
>>>
>>> - Confusion arises due to the fact that nullness (the absence
>>> of value) is often represented on computers by the number 0.
>>> (Obviously, 0 is not null.)
>>>
>>> - In some contexts, 'null' and 'nil' mean the same thing;
>>> in others, they do not.
>>
Date: Wed, 14 Feb 2007 02:43:18 GMT
Message-ID: <aHuAh.824$4c6.772_at_news-server.bigpond.net.au>
"mAsterdam" <mAsterdam_at_vrijdag.org> wrote in message news:45cdb22a$0$334$e4fe514c_at_news.xs4all.nl...
> mountain man quoted:
>>> [NULL]
>>> Roughly: a special marker that can be put in a place
>>> inside a data structure where an actual value is expected.
>>> Precisely what that marker means varies and there are at
>>> least three possibilities that are sometimes assumed:
>>>
>>> (1) "Unknown value" This means that on the place of the marker
>>> there should actually be a value but this value is not known
>>> at the present time. For example, if a 'name' field in a tuple
>>> describing a person is 'null' then this person will have a
>>> name but we don't know it.
>>>
>>> (2) "Absent value" This means that the property that is
>>> described by the value in question is simply not defined.
>>> For example, if the 'shipping-date' field in a tuple
>>> describing an order is 'null' then the order was
>>> not shipped yet.
>>>
>>> (3) "Whatever SQL says it means" The exact meaning is hard to
>>> summarize briefly, but is a mixture of the previous two
>>> interpretations and involves a value with three truth-values
>>> ('true', 'false' and 'unknown').
>>>
>>>
>>> Common usage:
>>>
>>> - Confusion arises when people use terms like "null value",
>>> a paradox to some, a contradictio in terminis to others.
>>>
>>> - Confusion arises due to the fact that nullness (the absence
>>> of value) is often represented on computers by the number 0.
>>> (Obviously, 0 is not null.)
>>>
>>> - In some contexts, 'null' and 'nil' mean the same thing;
>>> in others, they do not.
>>
<<<<=============
>> Everything looks good to this point. >> But I cant understand the meaning of the >> following sentence ....
>>> In databases traditionally NULL is used and and opposed. ***
>>> If you want to go into this, please first search for
>>> mu NIL void NULL undef, 2VL 3VL.
>> >> What does the first sentence mean? > > Could you elaborate? I really have difficulty seeing what might > not be understood here - maybe the phrasing could be improved a little: > "In the context of databases traditionally ..." ?
I thought the entry for NULL was good up to point above <<<===
Does this following sentence then read as follows:
"In the context of databases traditionally NULL is used and opposed."?
Just seeking clarification.
> I prefer not to participate in opening this NULL can of worms again. > NULL was discussed several times in c.d.t.; this is what came out. > However, if subsequent discussion will lead to someone > coming up with a better text (or better yet a reference) > for the complete NULL entry, I'll be happy to copy & replace it.
I agree.
>> BTW, this FAQ has been greatly expanded since the last time >> I read it, and represents a valuable resource --- ONYA. >> Well done. > > Thank you, on behalf of everybody who has contributed. > (I don't remember exactly - but that includes you, no? > -- not feeling like mining the archives right now) > > What does ONYA stand for? > http://acronyms.thefreedictionary.com/ONYA > says: "Oh No, You Again?" - I suspect I am missing > a joke/subtlety for native English speakers here.
Its an Australian abbreviation for "Good On You". Received on Wed Feb 14 2007 - 03:43:18 CET