one attribute referencing, attributes in two different tables - oracle

Question

I have 4 tables
customer: CustomerID - primary key, name
Magazine: name - primary key, cost, noofissues
Newspaper: name - primary key, cost, noofissues
subscription: custID - references CustomerID of Customer, name, startdate, enddate
In the above, can I reference the name from subscription table to reference name from Magazine and name from Newspaper?
I have created the tables Customer, Newspaper and Magazine. I only need to create Subscription.

Answer 1

Can you do something like this?
CREATE TABLE subscription (
custID INT
CONSTRAINT subscription__custid__fk REFERENCES Customer( CustomerId ),
name VARCHAR2(50)
CONSTRAINT subscription__mag_name__fk REFERENCES Magazine( Name )
CONSTRAINT subscription__news_name__fk REFERENCES Newspaper( Name ),
startdate DATE
CONSTRAINT subscription__startdate__nn NOT NULL,
enddate DATE
);
Yes, you can and you will have two foreign keys on the same column pointing to different tables but if the value in the column is non-null then it will expect there to be a matching name in both the magazines table and the newspapers table - which is probably not what you are after.
Can you have a foreign key that asks can the value be in either exclusively in this table or that table (but not in both)? No.
But you can re-factor your database so you merge the newspapers and magazines tables into a single table (which you can then easily reference); like this:
CREATE TABLE customer (
CustomerID INT
CONSTRAINT customer__CustomerId__pk PRIMARY KEY,
name VARCHAR2(50)
CONSTRAINT customer__name__nn NOT NULL
);
CREATE TABLE Publications (
id INT
CONSTRAINT publications__id__pk PRIMARY KEY,
name VARCHAR2(50)
CONSTRAINT publications__name__nn NOT NULL,
cost NUMBER(6,2)
CONSTRAINT publications__cost__chk CHECK ( cost >= 0 ),
noofissues INT,
type CHAR(1),
CONSTRAINT publications__type__chk CHECK ( type IN ( 'M', 'N' ) )
);
CREATE TABLE subscription (
custID INT
CONSTRAINT subscription__custid__fk REFERENCES Customer( CustomerId ),
pubID INT
CONSTRAINT subscription__pubid__fk REFERENCES Publications( Id ),
startdate DATE
CONSTRAINT subscription__startdate__nn NOT NULL,
enddate DATE
);

Answer 2

If you are asking whether you can create a foreign key constraint on subscription that references either the newspaper table or the magazine table, the answer is no, you cannot. A foreign key must reference exactly one primary key.
Since magazine and newspaper have the same set of attributes, the simple option is to combine them into a single periodical table with an additional periodical_type column to indicate whether it is a magazine or a newspaper. You could then create your foreign key to the periodical table.
Although it probably won't make sense in this particular example, you could also have separate columns in subscription for magazine_name and newspaper_name and create separate foreign key constraints on those columns along with a check constraint that ensured that exactly one of the values was non-NULL. That might make sense if the two different parent tables had radically different attributes.
Not related to your question but as a general bit of advice, I wouldn't use the name as the primary key. In addition to being rather long, names tend to change over time and names aren't necessarily unique. I would use a different attribute for the key, potentially a synthetic primary key generated from a sequence.