Re: Foreign superkey support


"Marshall" <marshall.spight@xxxxxxxxx> wrote in message
news:1155059837.979156.61820@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
paul c wrote:

Perhaps all I'm saying is that the basic relational algebra doesn't need
a foreign key concept which I admit is a deviation from the specific
question.

I don't think of constraints as being part of the algebra per se.
Constraints
are a useful mechanism for ensuring integrity of variables, whereas the
algebra is a way of constructing values from other values. (Including
the values in variables.) Note that the algebra is what you do queries
with, but constraints are relevant only when doing DML: insert, update,
delete. (Although we may use the algebra to construct values that
we then insert, for example.)


I have always thought that constraints were part of the predicate of the
database, and while their enforcement during DML operations is critical, I
don't think that their definition requires the concept of variables. A
database value is static, so any change involves selecting a different
database value. What constraints do is limit the scope of available
database values that can be selected. In other words, constraints narrow
the scope of the discussion.

It is quite interesting to consider the idea of constraints as
descriptive entities for values, in addition to being prescriptive
for variables. We can then consider propogation of these
descriptions through the algebraic operations.


Constraints not only limit the values that a attribute, tuple, relation or
database can take, but they also augment the meaning of the values that they
constrain. For example, if you have a relation that can only contain
statements about men, then any conclusion drawn from that applies only to
men or groups of men.


But as long as people like the shortcut notation, I think the
example is a good reason for allowing super foreignkeys or foreign
superkeys!

Frankly I think there's too much emphasis on foreign keys. I
think we should emphasize the use of domestic keys wherever
possible, and use foreign keys only when the domestic variety
is not available.

I don't think there's anything wrong with foreign keys, as long as they're
not illegal.

Marshall

PS. dumb joke, sorry.



.

Relevant Pages

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