Adding foreign key constraint to associative entity - oracle

Question

I have 3 tables:
1) SERVICE_REQUESTS (Parent)
CREATE TABLE SERVICE_REQUESTS(
service_id NUMBER (7) NOT NULL,
serial_number NUMBER (10) NOT NULL,
service_date DATE NOT NULL,
service_description VARCHAR(50) NOT NULL,
hourly_rate NUMBER(5,2) NOT NULL,
customer_id NUMBER (5) NOT NULL,
employee_id NUMBER (3) NOT NULL,
CONSTRAINT service_request_pk PRIMARY KEY(service_id, serial_number));
2) SERVICE_PARTS (Associative Entity to solve M:M relationship)
CREATE TABLE SERVICE_PARTS(
service_id NUMBER (7) NOT NULL,
part_id NUMBER (10) NOT NULL,
quantity NUMBER (4) NOT NULL,
unit_cost NUMBER(7,2) NOT NULL,
CONSTRAINT service_part_pk PRIMARY KEY(service_id, part_id));
3) PARTS (Parent)
CREATE TABLE PARTS(
part_id NUMBER (10) NOT NULL,
part_description VARCHAR(50) NOT NULL,
cost NUMBER(7,2) NOT NULL,
quantity_on_hand NUMBER (5) NOT NULL,
CONSTRAINT part_pk PRIMARY KEY(part_id));
I've created a foreign key constraint from SERVICE_PARTS to PARTS with the following statement:
ALTER TABLE service_parts
ADD CONSTRAINT service_parts_part_id_fk
FOREIGN KEY (part_id)
REFERENCES parts(part_id);
Now I'm trying to create a foreign key constraint from SERVICE_PARTS to SERVICE_REQUESTS using the follow statement:
ALTER TABLE service_parts
ADD CONSTRAINT service_parts_service_id_fk
FOREIGN KEY (service_id)
REFERENCES service_requests(service_id);
But I get the following error: ORA-02270: no matching unique or primary key for this column-list. Why does it allow a constraint to be added for the part_id but not the service_id?
I've attached my ER Diagram for visual clarification:

Answer 1

If the primary key of the service_requests table is service_id, serial_number, your M:M mapping table would need to include both elements of the primary key. The definition of the primary key implies that you can have many rows with the same service_id but different serial_number values. If your mapping table doesn't contain both elements of the key, you wouldn't be able to figure out which particular row in service_requests was mapped to any particular row in service_parts.
Your mapping table definition would need to be
CREATE TABLE SERVICE_PARTS(
service_id NUMBER (7) NOT NULL,
serial_number NUMBER (10) NOT NULL,
part_id NUMBER (10) NOT NULL,
quantity NUMBER (4) NOT NULL,
unit_cost NUMBER(7,2) NOT NULL,
CONSTRAINT service_part_pk PRIMARY KEY(service_id, serial_number, part_id));
and then your foreign key
ALTER TABLE service_parts
ADD CONSTRAINT service_parts_service_id_fk
FOREIGN KEY (service_id, serial_number)
REFERENCES service_requests(service_id, serial_number);
Alternately, if we believe that your mapping table is correct, which would imply that service_id was the key for service_requests, then the definition of service_requests would be
CREATE TABLE SERVICE_REQUESTS(
service_id NUMBER (7) NOT NULL,
serial_number NUMBER (10) NOT NULL,
service_date DATE NOT NULL,
service_description VARCHAR(50) NOT NULL,
hourly_rate NUMBER(5,2) NOT NULL,
customer_id NUMBER (5) NOT NULL,
employee_id NUMBER (3) NOT NULL,
CONSTRAINT service_request_pk PRIMARY KEY(service_id));
Then your mapping table and your foreign key definition would be correct as is.

Answer 2

Your primary key in table SERVICE_REQUESTS:
CONSTRAINT service_request_pk PRIMARY KEY(service_id, serial_number)
Such primary key can't guarantee, that your service_id value will be unique. Hence, you can't make reference only to this field from another table, because there is no way to determine, to which record you will refer. Your primary key in one table and foreign key in another have to be exactly the same.