On Oct 27, 2006, at 10:38 AM, Paul McNett wrote:
Using the invoice & customer tables example,
the invoice table has an iClientId column that points
at the clients PK, iid. So iClientId is a foreign
key. I would think invoice would be the parent and
client would be the child -- do you agree or disagree?
Disagree. There's one customer to many invoices, not one invoice to
many customers.
Parent/child is an unfortunate choice of terms. Typically in
designing database systems, the better terminology is independent/
dependent.
The choice of which is which depends on the particular process. If
you are looking at a list of customers, the set of selected invoices
is dependent on the chosen customer. That is why the invoice is
considered a child of customer.
However, imagine a module in that app in the shipping department.
They get an invoice, and need to see the related information. In that
case, the customer record that is displayed is dependent on the
selected invoice. Select a different invoice; get a different customer.
The choice of how to link related tables depends on the nature of
the relationship. For 1:M relations, it makes sense for the 'many'
table to have a link to the single record it is related to. For M:M
tables, neither has a foreign key to the other; those are maintained
in separate tables typically called 'allocation' or 'relation'
tables. For 1:1 relations, it really doesn't matter, but typically if
one table is considered 'primary' and the other 'supplemental', the
supplemental table contains a FK linking back to the main.
-- Ed Leafe
-- http://leafe.com
-- http://dabodev.com
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