Re: Date, Darwen, Pascal and the alternative to Nulls in the RM
- From: paul c <toledobythesea@xxxxxxxx>
- Date: Thu, 23 Mar 2006 20:56:15 GMT
David Cressey wrote:
"falcon" <shahbazc@xxxxxxxxx> wrote in message
news:1143134130.259128.181350@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
...
Notice the missing [222-22-2222, joe, null, C] entry since this tuple
doesn't evaulate to TRUE (due to the null), it can not be part of any
result set.
I'm with you up to this point. Notice that the tuple you flag as missing is
missing because you preformed an INNER join. See below.
...
Although I haven't read the SQL ANSI/ISO Standard (hard to believe that something so big could possibly be coherent, even snippets of it seem to confirm this), I have read elsewhere that it doesn't mention the word 'relation' (which may be to its credit in a half-assed way). Don't see how SQL creations such as 'INNER join' could involve tuples.
pc
.The missing entry for 'joe' in the last query might be very difficult
for someone coming from an sql background, working at a real company.
I understand that logical models should not worry about 'real-world'
matters. As far as I can see, this problem of missing joe can be
mitigated by allowing something similar to outer joins when the
result-set does indeed include a null, such as follows:
[111-11-1111, jim, 21, A]
[222-22-2222, joe, null, C]
[333-33-3333, jack, 23, B]
Yes. Coming from and SQL background, my reaction would be "what the hell
did they do with the outer join?" Once they provide an outer join (to
satisfy information requirements), it will give you the answer you outlined
above, null and all.
But then, this is not a relation. In other words, the previous three
lines may appear on a user's computer screen, but they can no longer be
part any further relational operations.
Too bad. This is one of the differences between relational algebra and the
relational data model. The model was closely based on the algebra, with due
attention to detail. But the relational data model is a DATA model, and not
a mathematical model.
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