Problems referencing primary key with foreign key - sql

Question

This is code to create the beginnings of a book library database with Microsoft SQL Server Management Studio.
CREATE DATABASE BOOK_LIBRARY
CREATE TABLE LIBRARY_USER
(
usr_id int not null primary key,
f_name varchar(30) not null,
m_init char(1),
l_name varchar(30) not null,
balance decimal(6,2),
join_date date,
addrss_1 varchar(30) not null,
addrss_2 varchar(30),
city varchar(30) not null,
addrss_state char(2) not null,
zip_code varchar(10) not null,
email varchar(30)
);
CREATE TABLE LIBRARY_TRANSACTIONS
(
transaction_id int not null primary key,
maximum_borrow_duration int not null,
strt_date date not null,
actual_return_date date not null,
borrow_usr_id int not null,
foreign key (borrow_usr_id) references LIBRARY_USER(usr_id)
);
CREATE TABLE BOOKS
(
isbn varchar(17) not null primary key,
title varchar(30) not null,
number_of_copies int not null,
Author varchar(30) not null,
Number_of_pages int not null,
publish_year int not null,
book_type varchar(20)
);
CREATE TABLE DIGITAL_BOOKS
(
digital_id int not null primary key,
format varchar(30) not null,
size_mb int not null,
digital_isbn varchar(17) not null,
foreign key(digital_isbn) references BOOKS(isbn)
);
CREATE TABLE PHYSICAL_BOOKS
(
physical_id int not null primary key,
condition varchar(20) not null,
physical_isbn varchar(17) not null,
foreign key(physical_isbn) references BOOKS(isbn)
);
CREATE TABLE BOOK_COPY
(
digi_id int not null,
phys_id int not null,
primary key(digi_id, phys_id),
foreign key(digi_id) references DIGITAL_BOOKS(digital_id),
foreign key(phys_id) references PHYSICAL_BOOKS(physical_id)
);
CREATE TABLE CONTNS
(
trans_id int not null primary key,
digi_id int not null,
phys_id int not null,
foreign key(digi_id) references BOOK_COPY(digi_id),
foreign key(phys_id) references BOOK_COPY(phys_id)
);
Despite me being able to look and see that digi_id and phys_id are in fact primary keys of the book_copy table, I keep getting this error:
Msg 1776, Level 16, State 0, Line 66
There are no primary or candidate keys in the referenced table 'BOOK_COPY' that match the referencing column list in the foreign key 'FK__CONTNS__digi_id__37A5467C'.
Msg 1750, Level 16, State 1, Line 66
Could not create constraint or index. See previous errors.
Am I missing something obvious? I'm just starting out with using this program, so any help is greatly appreciated.

Answer 1

Here:
CREATE TABLE CONTNS
(
trans_id int not null primary key,
digi_id int not null,
phys_id int not null,
foreign key(digi_id) references BOOK_COPY(digi_id),
foreign key(phys_id) references BOOK_COPY(phys_id)
);
You are creating two foreign keys to parent table BOOK_COPY. But that table has a compound primary key (ie a multi-column primary key)
CREATE TABLE BOOK_COPY
(
digi_id int not null,
phys_id int not null,
primary key(digi_id, phys_id), --> here
...
)
As a consequence, you need a compound foreign key:
CREATE TABLE CONTNS
(
trans_id int not null primary key,
digi_id int not null,
phys_id int not null,
foreign key(digi_id, phys_id) references BOOK_COPY(digi_id, phys_id)
);