Stephen
That more of an RI/Cardinality rule than a business rule, this is
achievable with a composite Candidate Key it does not have to be the PK
Neven
I find it useful to use a composite PK when I want to get the database
to enforce business rules. For example, to take Anru's example above
of the [order]>--<[line] many-to-many relationship, you are left with
the problem of how to manage adding product lines to the order.
You could do this:
query table WHERE order_id=order AND product_id=product
no rows returned?
INSERT row and set order_id=order AND product_id=product
else
UPDATE existing row
But I prefer to use the database engine to do my dirty work. Put the
PK on order_id + product_id, then:
INSERT row and set order_id=order AND product_id=product
Trap for "Would create duplicate values and violate Primary Key"
error
In errorhandler:
UPDATE existing row
Hope this example makes sense. Perhaps other members could comment
whether this is a favoured practice or not?
-stephen/.
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