Misa Simic wrote
> I guess that rule can be achieved with triigers on TableA and TableC - but
> the same is true for FK (and FK constraint is more effective then trigger
> -
> that is why I wonder would it be useful/achievable to create that kind of
> constraint)
>
> Thoughts, ideas?
You create a common "keys in use" table and only insert a record into the
main tables if you can successfully add the desired key to the shared keys
table ( as a unique value ). Setup a normal FK to that table to help
enforce that valid records must exist on the keys table. Not fool-proof but
you only need to worry about insertions - delete from the pk table to remove
the record from the main table and free up the key.
David J.
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