Re: General ISA and Foreign Key

BOUCHPAN-LERUST-JUERY Lionel wrote:
>In SQL I have the following tables
>
>CREATE TABLE film(
>VisaExploitation INTEGER NOT NULL,
>DureeTournage INTEGER NOT NULL,
>Titre VARCHAR( 50 ),
>
>PRIMARY KEY ( VisaExploitation ) );
>CREATE TABLE filmHistorique(
>NbCostume INTEGER
>) INHERITS ( film );
>
>create table filmDocumentaire(
> ) INHERITS ( film );
>
>I have a weak entity:
>
>CREATE TABLE copie(
>NumCopie INTEGER NOT NULL,
>VisaExploitation INTEGER NOT NULL,
>PRIMARY KEY( VisaExploitation, NumCopie ),
>FOREIGN KEY( VisaExploitation ) REFERENCES film ON DELETE CASCADE );
>
>The problem is I have to be able to have the constraint on
>both 2 and 3 and I can't figure how to implement this.

You can't do this at present, because there is no support for a foreign
key constraint on an inheritance hierarchy (it is a major defect with the
current implementation of inheritance).

Although inheritance is conceptually correct, the lack of implementation
suggests an alternative course:

"film" should contain a row for every film and "filmHistorique" and
"filmDocumentaire" should have foreign key constraints on "film". You
could maintain the contents of "film" by triggers on the other two tables.

--
Oliver Elphick Oliver(dot)Elphick(at)lfix(dot)co(dot)uk
Isle of Wight http://www.lfix.co.uk/oliver
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