Number of referencing columns must match referenced columns - sql

Question

I am relatively new to SQL and I keep getting the following error "The number of columns in the foreign-key referencing list is not equal to the number of columns in the referenced list."
create table client
(name varchar(30),
phone int,
city varchar(20),
state char(2) CHECK(state='MN' OR state='ND' OR state='SD' OR state='WI' or state='IA'),
primary key(name,phone));
create table owns_vehicle
(name varchar(30),
phone int,
vin varchar(10),
primary key(name, phone, vin),
foreign key(name,phone) references client);
create table service_appointment
(mydate date,
vin varchar(10),
mechanic varchar(15),
description varchar(30) NOT NULL,
cost int CHECK (cost>=0),
primary key(mydate,vin),
foreign key(vin) references owns_vehicle);
This is the line that is causing the issue:
foreign key(vin) references owns_vehicle);
Does anyone know why I am getting this error?

Answer 1

You try to reference owns_vehicle, thus you need to reference all of the tables PRIMARY KEY columns, which would be name, phone, vin. But your foreign key has only vin, thus there are two columns (name, phone) missing.

Answer 2

The primary key has three parts. You need to reference all of them. If you are going to use this model, then you need the other two components:
create table service_appointment (
mydate date,
name varchar(30),
phone int,
vin varchar(10),
mechanic varchar(15),
description varchar(30) NOT NULL,
cost int CHECK (cost>=0),
primary key(mydate,vin),
foreign key(name, phone, vin) references owns_vehicle (name, phone, vin)
);
However, I think you data model should change. VINs can change owners. People can change phone numbers. These don't seem like good components of a primary key.

Answer 3

If you assume that a vehicle can only have a single owner at a given time, you can make your current data model work by changing owns_vehicle to have a uniqueness constraint on VIN:
CREATE TABLE OWNS_VEHICLE
(NAME VARCHAR2(30),
PHONE INT,
VIN VARCHAR2(10),
CONSTRAINT PK_OWNS_VEHICLE
PRIMARY KEY(NAME, PHONE, VIN)
USING INDEX,
CONSTRAINT OWNS_VEHICLE_UQ1
UNIQUE(VIN)
USING INDEX,
CONSTRAINT OWNS_VEHICLE_FK1
FOREIGN KEY(NAME,PHONE) REFERENCES CLIENT(NAME, PHONE));