RI and NULL's

One more question:

    I'm planning to create generic trigger procs for PK/FK stuff.
    So that it's simply insert/delete the appropriate  pg_trigger
    entries during CREATE/ALTER table.

    Assuming  NULL's  are  allowed  in FK values (are they?), I'd
    like to know what the correct handling of NULL values is.  If
    an  attribute  of the FK has the NULL value, must a PK with a
    NULL  in  the  corresponding  attribute  exist  or  is   this
    attribute  completely  left  out  of  the WHERE clause in the
    check?

    Other way round -  NULL  value  in  attribute  of  referenced
    table.   What  to  delete  from  FK  in the case of ON DELETE
    CASCADE?


Jan

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