[Techtalk] Relational databases?

[David Merrill]

On Thu, Sep 06, 2001 at 04:48:46PM -0500, Jenny Brown (was Gable) wrote:
> On Thu, 6 Sep 2001, Kath wrote:
> 
> > What is it exactly?  I think I understand the concept, but I'm not sure.
> > Is it like an actual symlink or is it more like "Okay, we have a product id 
> > for this product, and we will use it through out our database here in our 
> > products table, here in our customer shipping table, here in our orders 
> > from supplier table and here in the user's shopping cart"?
> 
> > Is there a specific command you use in MySQL to make something relational 
> > or is it just an idea or practice?
> 
> You have the concept correct with the consistent use of IDs.  A database
> can enforce 'referential integrity' by not allowing data to be added
> that "doesn't make sense" in the context of the rules you set up.  A
> database is still considered relational even if it doesn't enforce 
> integrity, as long as whatever is using it does so sensibly.
> 
> A "primary key" is an id that uniquely identifies one row; in practice
> this is things like, customer id, product serial number, etc.  A
> "foreign key" is when you use a primary key in another table.  For
> instance, I might use customer id in the customer table, with name, 
> address, phone, email; in that case it's a primary key.  Then in a
> separate table I might list a region, with what customers are in that
> region; in that case, the region id would be a primary key, and the
> customer id list would be foreign keys (since they refer to a foreign
> table).
> 
> You can run commands in sql to define primary and foreign keys, and
> the behavior when the data is changed. For instance, if I delete
> customer #352, I could choose 'cascading delete': all records in
> other tables that refer to that same customer would also be deleted.
> This way I wouldn't have any orphaned data.
> 
> A primary or foreign key is typically called a 'constraint' in the
> database; you can find documentation of the commands by searching
> for the words primary key and constraint.
>
> In practice not all relational databases actually enforce integrity,
> in order to allow for flexibility in programming; but often the
> software that creates and uses the data does the integrity checks instead.

Then that isn't a relational database.

A relational database platform in the practical sense, and most common
usage, is one which provides the tools for enforcing referential
integrity. While Oracle is a relational database platform (i.e.,
provides all the tools for building referential integrity into your
specific database application), you can still create tables in it that
don't use those features. That would be a non relational database
built on a relational database platform.

Please note the distinction between a database platform (e.g., Oracle)
and a specific database (e.g., your billing system). People use the
word `database' for both, and that can cause confusion.

Now back to my Oracle programming...

-- 
Dr. David C. Merrill                     http://www.lupercalia.net
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