Anthony W. Youngman skrev:
At which point, you hit my hobbyhorse ... "In the real world ..." -
relational database theory has ABSOLUTELY NOTHING whatsoever to do
with the real world. It's an exercise in pure maths.
And You hit my hobbyhorse or rather one of them :-) ...
RDMS theory is absolutely not pure maths!
In pure maths a relation is the set of tuples (ordered lists of n
elements {e1,e2,e3...en }
where the first element e1 belongs to a set S1 and e2 to set S2 etc.
and there are no limitations on the nature of theses sets.
Not even that the elements must be atomic - in contrary they may be sets
themselves
or lists ( like our multivalues ) or even relations ( like our
associations ).
Pure maths doesn't limit the nature of the defining sets at all -
a set consisiting of my car, the north pole,the number pi and the set of
all real numbers is valid.
Thats a long way from rdbms integers,date, fixed length strings etc..
.
The explicit goal of Codds rules ( at least in the paper where I read them)
is to limit this to something that is possible to handle on
computer hardware (of the 70-ies!?) as well as allowing a relative
easy formulation of select criteria,
constraints etc.
From an u2 or pick view Codd choose to include unecessary contraints on
relations
for the RDBMS,
But that doesnt make it more or less mathematical than the pick model..
In my maths class we only had this definiton of an relation and then
emediately
went on to study relations where the all the attributes where real
numbers so the
general defintion may be to much even for mathematicians to cope with....
-- mats
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