Database Design Advice
Smylers at stripey.com
Thu Nov 7 16:02:37 GMT 2013
William Blunn writes:
> On 07/11/2013 13:03, Smylers wrote:
> > A purchase consists of ordering one product from a supplier. Each
> > product is only available from a single supplier. So a record in
> > the purchase table just needs to store a product ID, and by
> > linking through the product table that defines the supplier too.
> > A purchase will also be handled by a particular contact at the
> > supplier. Again, the purchase table can link to the
> > supplier_contact table, which in turn links to the supplier.
> > Except there's now two (indirect) links from the purchase table to
> > the supplier table. Can the database enforce that they both go to
> > the same supplier? (That is, that a purchase handled by a
> > particular contact must be for a product sold by that contact's
> > company?)
> I assume your purchase table has foreign key constraints
Yeah, I'm assuming that too! (Currently this database only exists on
> product_id -> product (id)
> contact_id -> contact(id)
> And you want to ensure that for a given purchase row that
> product(supplier_id) = contact(supplier_id)?
> You could add supplier_id to the purchase table.
> Now this will create a denormalisation. To ensure that your data
> does not become inconsistent, you need to extend your foreign key
> constraints on the purchase table to be:
> (product_id, supplier_id) -> product(id, supplier_id)
> (contact_id, supplier_id) -> contact(id, supplier_id)
Makes sense — thank you for the suggestion.
> I used this approach in anger at a former employer and it worked well.
Good to know.
I'm now reading up on window functions, prompted by your other mail.
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