Add constraints for a foreign key that references multipleprimary keys from different tables in SQL Plus? - sql

Question

I'm a beginner learning SQL and am having trouble implementing this concept.
Suppose you create the following three tables:
CREATE TABLE dogOwner(
ownerNo VARCHAR(8) CONSTRAINT ownerNo_pk1 PRIMARY KEY,
ownerName VARCHAR(10)
);
CREATE TABLE catOwner(
ownerNo VARCHAR(8) CONSTRAINT ownerNo_pk2 PRIMARY KEY,
ownerName VARCHAR(10)
);
CREATE TABLE petsAdopted(
petNo VARCHAR(8) CONSTRAINT petNo_pk PRIMARY KEY,
ownerNo VARCHAR(8) CONSTRAINT ownerNo_fk1 REFERENCES dogOwner(ownerNo)
CONSTRAINT ownerNo_fk2 REFERENCES catOwner(ownerNo)
);
How do you properly create constraints for the foreign key ownerNo, which references ownerNo from two other tables?

Answer 1

You can't. You could have 2 columns in petsAdopted: dogOwnerNo and catOwnerNo and 2 foreign keys. But the table design doesn't seem to make sense: surely a pet either is a dog or a cat (or something else) regardless of who owns it?
Here is an alternative design:
CREATE TABLE owner(
ownerNo VARCHAR(8) CONSTRAINT ownerNo_pk2 PRIMARY KEY,
ownerName VARCHAR(10)
);
CREATE TABLE petsAdopted(
petNo VARCHAR(8) CONSTRAINT petNo_pk PRIMARY KEY,
petType VARCHAR2(10) NOT NULL CONSTRAINT petTypeChk (CHECK petType in ('CAT','DOG'))
ownerNo VARCHAR(8) CONSTRAINT ownerNo_fk REFERENCES owner(ownerNo)
);

Answer 2

This only address syntax part of your answer which You have got lot of syntax error, but this will not work, Please rethink your design.
CREATE TABLE dogOwner(
ownerNo VARCHAR(8) CONSTRAINT ownerNo_pk1 PRIMARY KEY,
ownerName VARCHAR(10)
);
CREATE TABLE catOwner(
ownerNo VARCHAR(8),
ownerName VARCHAR(10),
CONSTRAINT ownerNo_pk2 PRIMARY KEY (ownerNo),
CONSTRAINT ownerNo_fk1 FOREIGN KEY (ownerNo) REFERENCES dogOwner(ownerNo)
);
CREATE TABLE petsAdopted(
petNo VARCHAR(8) ,
ownerNo VARCHAR(8),
CONSTRAINT petNo_pk PRIMARY KEY (petNo),
CONSTRAINT ownerNo_fk_pet1 FOREIGN KEY (ownerNo) REFERENCES dogOwner(ownerNo),
CONSTRAINT ownerNo_fk_pet2 FOREIGN KEY (ownerNo) REFERENCES catOwner(ownerNo)
);