The Third Manefesto

I'm almost done reading Date's book. I have a bunch of questions and need to reread and think about a number of sections. There is one basic question, though, where I am convinced I must be missing something.

How do you apply Foreign Keys if relations are defined using relational types?

One example given by Date is logically representing employee data either in a relation variable (EMP) with attributes empno, ename, deptno, ... OR by defining a type emp(empno, ename, deptno, ...). This type is then the data type (domain) of an EMP relvar. The second relation is thus effectively a set of employee objects.

I see how joins can be done in the second case (use a built-in funtion THE_DEPTNO(emp) which will extract the deptno value from each tuple), but what about a foreign key on DEPTNO?

If I read this in the morning and the question is as unclear as I suppose it is, I'll expound on this.

Henry

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