Creating foreign keys from entites that are located in separate tables - sql

Question

I've been scratching my head at this for hours.
I have four tables Book, Author, Allocation and WorkSession
I need to make a constraint that the combination of the bid and authID exists in the Allocation table from the WorkSession table.
I tried to create a foreign key constraint like you would normally do but it won't work because the id's referenced are both foreign keys already and I can't create a new key as an identifier for the allocations table
CREATE TABLE Book(
bid NUMBER(4),
title VARCHAR2(30) NOT NULL,
SellingPrice NUMBER(6,2),
PRIMARY KEY (bid),
CONSTRAINT EM_SellingPrice CHECK(SellingPrice >= 0)
);
CREATE TABLE Author(
authID NUMBER(4),
fName VARCHAR2(30),
sName VARCHAR2(30),
PRIMARY KEY(authID),
CONSTRAINT EM_Name UNIQUE (fName,SName)
);
CREATE TABLE Allocation(
bid NUMBER(4),
authID NUMBER(4),
payRate NUMBER(6,2),
CONSTRAINT AL_PayRate CHECK(payRate >= 1 AND payRate < 80),
FOREIGN KEY(bid) REFERENCES Book(bid),
FOREIGN KEY(authID) REFERENCES Author(authID)
);
CREATE TABLE WorkSession(
bid NUMBER(4),
authID NUMBER(4),
WorkYear NUMBER(4),
WorkWeek NUMBER(2),
WorkHours NUMBER(4,2),
CONSTRAINT WY_Range CHECK(WorkYear > 2010 AND WorkYear < 2014),
CONSTRAINT WW_Range CHECK(WorkWeek >= 1 AND WorkWeek <= 52),
CONSTRAINT WH_Range CHECK(WorkHours >= 0.5 AND WorkHours <100),
CONSTRAINT FK_Check FOREIGN KEY(bid, AuthID) REFERENCES Allocation(bid, AuthID),
PRIMARY KEY(workYear, WorkWeek)
);

Answer 1

CREATE TABLE Allocation(
bid NUMBER(4),
authID NUMBER(4),
payRate NUMBER(6,2),
CONSTRAINT AL_PayRate CHECK(payRate >= 1 AND payRate < 80),
FOREIGN KEY(bid) REFERENCES Book(bid),
FOREIGN KEY(authID) REFERENCES Author(authID)
);
A foreign key constraint has to target something unique. You can do that with either a PRIMARY KEY constraint or a UNIQUE constraint on one or more columns. Your table "allocation", above, doesn't have either of those constraints.
If {bid, authid} are unique in the table "allocation", then you can declare PRIMARY KEY (bid, authid) in that table. A foreign key that REFERENCES allocation (bid, authid) should work then.