ForeignKey-Constraint with more than 1 tables

Hi ng,

(Oracle 9.2.0.7i)

I talk about a database concerning real Trees. There is a table TTrees.

A single Tree belongs either to a forrest or to a street or to a park or whatelse.

So there exist f.e. a table TStreets and a table TForrests. in the table TTrees actually exist a field for each Table, f.e.

CIDFORREST
CIDSTREET Exactly 1 of them is not NULL, all the others are definitely NULL.

Now my Questions:
Is there a way to substitute this by only 1 Field, f.e. CID_BELONGS_TO _AND_ to build a FK-constraint like

TForrests.CID=TTrees.CID_BELONGS_TO OR
TStreets.CID=TTrees.CID_BELONGS_TO ...
?

Is there a possibility to build a constraint, that observes that _exact_ 1 of the fields (CIDFORREST, CIDSTREET ..) is not null? (I know, I could use triggers for it)?

Is there a better way to build this database? (The Columns of TTrees are 95% equal, no matter if it stands on a street or in a forrest, so there is only 1 table)

Many thanks
Andreas Mosmann

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