Re: Access as a RDBMS--why the multiple relationships?
Date: Fri, 14 Dec 2007 21:40:24 -0000
"raylopez99" <raylopez99_at_yahoo.com> wrote in message
> OK, this is about my fifth day doing databases, and I've read about
> 1NF, 2NF, 3NF here:
> and I have a crappy book by a guy named Louis Davidson (APress), full
> of typos, bloated, and light on theory, but here's my question
> concerning Microsoft Access.
> I notice in Microsoft Access the relationship chart can, unlike most
> textbooks, have TWO, not just one, relationship arrows between
> tables. But I think (and I just want confirmation of this) that one
> of these two relationship arrows is bogus, and more like a query than
> a true relationship.
> Here goes:
> relationship arrow one (sorry I can't do the SQL statement thing from
> my head, like some of you can, so I'll use prose instead):
> entity TABLE A has Primary Key PKA that migrates to entity TABLE B as
> a (manditory, and non-exclusive, but doesn't matter) Foreign Key FKB.
> In the "relationship diagram" (Access likes to use diagrams), this is
> properly shows as a 1 to infinity symbol, no problem, this is also in
> my textbook.
> But I also using Access can set up ANOTHER SECOND?! relationship
> comprising a non-primary, non-key, non-unique field in TABLE A with a
> non-unique field in TABLE B. In the relationship diagram this shows
> up as "indeterminite" (no infinite or numbered symbols) and further
> you cannot check the box for "enforce referential integrity" because
> you get the error message (which makes sense) of "No unique index
> found for the referenced field of the primary table" (since the
> referenced field is not a primary key, or any kind of key).
> I just want confirmation that the SECOND relationship above is simply
> a bogus construct of Access, akin to a query constraint of some sort,
> and not really a 'relationship' as defined by RDBMS theory.
A relationship in an RDBMS is represented in only one way: as values within tuples within relations. That is all relational theory has to say about it.
Access is an application development tool and file sharing program that incorporates a poor imitation of a SQL (not relational) data model. Any pictures it draws have little to do with relational theory and I don't think you should expect to learn anything useful from them. A picture of a thing is not the same as a thing.
-- David PortasReceived on Fri Dec 14 2007 - 22:40:24 CET