我的查询在应该抛出错误时继续

Question

So I am trying to validate a query

The below code, should throw an error. Because VIN '2234' belongs to user with ID of 2, why it doesn't throw an error when I am trying to add this VIN to user with ID of 1?

Each car has an unique VIN and a client id. Whenever I add a service with a specific client the client id must match his VIN number.

create table client (
    id BIGSERIAL primary key,
    name VARCHAR(20)
);

create table cars (
   vin VARCHAR(20) not null unique primary key,
   fk_client BIGSERIAL unique,
   FOREIGN KEY (fk_client) references client(id)
);

create table service (
    id BIGSERIAL primary key,
    fk_client BIGSERIAL,
    fk_vin VARCHAR(20),
    FOREIGN KEY(fk_client) references cars(fk_client),
    FOREIGN KEY(fk_vin) references cars(vin)
);

insert into client(name) values ('Jan');
insert into client(name) values ('John');

insert into cars (vin, fk_client) values ('223', 1);
insert into cars (vin, fk_client) values ('2234', 2);

insert into service (fk_client, fk_vin) values (1,'2234');//should throw error, because VIN 2234 doesn't belong to user with ID of 1.

Answer 1

When a service is for a car, we know the client, because the car row references a client row.

So, if every service is for a car, there must be no client ID in the service table.

The only reason for storing a client ID in the service table would be that there are not only services for cars, but also services for clients regardless of a car. In that case you'd either store the car ID or the client ID in the service table. What you don't want is a row with both a client and a car or neither a car nor a client. You can add a check constraint to ensure that always one of the IDs is set:

ALTER TABLE service
ADD CONSTRAINT chk_service_ids
CHECK ( (fk_client IS NOT NULL AND fk_vin IS NULL) OR 
        (fk_client IS NULL AND fk_vin IS NOT NULL) );

So, depending on what you really want, either delete the client ID from the table or add the check constraint.