Cant create tables, sql error ORA-02270 - sql

Question

Yeah, not too sure on this one. New to sql and I thought everything was done right, any ideas? If this is not the way to add foreign keys, could someone please explain to me how the correct way to do it is please? Would appreciate it, thanks.
CREATE TABLE customer (
reference NUMBER(5) PRIMARY KEY,
company_name VARCHAR2(30),
address VARCHAR2(30),
post_code VARCHAR2(10),
telephone VARCHAR(20),
contact_fname VARCHAR2(20),
contact_sname VARCHAR2(20),
contact_email VARCHAR2(30)
);
CREATE TABLE manifest (
barcode NUMBER(10) PRIMARY KEY,
trip_id NUMBER(10),
pickup_customer_ref VARCHAR2(30),
delivery_customer_ref VARCHAR2(30),
category NUMBER(1),
weight NUMBER(10)
);
CREATE TABLE category (
category NUMBER(1) PRIMARY KEY,
description VARCHAR2(15),
requirements VARCHAR2(30),
FOREIGN KEY (category) REFERENCES manifest(category)
);
CREATE TABLE trip (
trip_id NUMBER(10) PRIMARY KEY,
departure_date DATE,
return_date DATE,
vehicle_id VARCHAR2(10),
employee_no NUMBER(10),
FOREIGN KEY (trip_id) REFERENCES manifest(trip_id)
);
CREATE TABLE vehicle (
registration VARCHAR2(10) PRIMARY KEY,
vehicle_type_id VARCHAR2(10),
model VARCHAR2(15),
make VARCHAR2(15),
body VARCHAR2(15),
year NUMBER(4),
FOREIGN KEY (registration) REFERENCES trip(registration)
);
CREATE TABLE model (
vehicle_type_id VARCHAR(10) PRIMARY KEY,
make VARCHAR2(15),
model VARCHAR2(15),
FOREIGN KEY (vehicle_type_id) REFERENCES vehicle(vehicle_type_id)
);
CREATE TABLE driver (
employee_no NUMBER(10) PRIMARY KEY,
first_name VARCHAR2(20),
last_name VARCHAR(20),
ni_no VARCHAR2(15),
telephone VARCHAR2(20),
mobile VARCHAR2(12),
hazardous_goods VARCHAR2(1),
FOREIGN KEY (employee_no) REFERENCES trip(employee_no)
);
and the error message I get is
SQL Error: ORA-02270: no matching unique or primary key for this column-list
02270. 00000 - "no matching unique or primary key for this column-list"
*Cause: A REFERENCES clause in a CREATE/ALTER TABLE statement
gives a column-list for which there is no matching unique or primary
key constraint in the referenced table.
*Action: Find the correct column names using the ALL_CONS_COLUMNS
catalog view
I get this error for every table after manifest btw

Answer 1

The error message is pretty clear. The reference for a foreign key needs to be a unique or primary key in the other table. So this is wrong in the category table.
FOREIGN KEY (category) REFERENCES manifest(category)
I assume you intend for the manifest to look like this:
CREATE TABLE manifest (
barcode NUMBER(10) PRIMARY KEY,
trip_id NUMBER(10) REFERENCES trip(trip_id),
pickup_customer_ref VARCHAR2(30),
delivery_customer_ref VARCHAR2(30),
category NUMBER(1) REFERENCES category(category),
weight NUMBER(10)
);
And so on for the other places where foreign keys are used. (This uses a short-hand notation for the foreign key reference; a separate clause in CREATE TABLE is also fine.)
Of course, the definition of manifest has to go after category and trip, in this example.
In other words, the reference goes in the other table, not where the primary key is defined.
Also, I would recommend being consistent with the names of the foreign keys. At the very least, they should generally have the same name as the primary key, where possible.

Answer 2

trip_id, vehicle_type_id, and employee_no all need to be marked as UNIQUE.