Foreign key referring to multiple rows in another table

问题

I am working with MySQL, trying to build an online vehicles database system. For this purpose, I created two tables: vehicles and owners.

create table vehicles
(
  v_id int AUTO_INCREMENT PRIMARY KEY,
  v_reg_number varchar(255) unique,
  v_engine_number varchar(255) unique,
  v_chassis_number varchar(255) unique,
  v_type varchar(255),
  v_manufacturer varchar(255),
  v_model_year varchar(255),
  v_power varchar(255),
  v_origin varchar(255) 
)

create table owners
(
  o_id int AUTO_INCREMENT PRIMARY KEY,
  v_id int,
  o_name varchar(255),
  o_father_name varchar(255),
  o_cnic varchar(255) unique,
  o_dob varchar(255),
  o_gender varchar(255),
  FOREIGN KEY(v_id) REFERENCES vehicles(v_id)
)

So one owner may have more vehicles registered in his name. My question is how can multiple IDs, referring to the vehicles table, be saved in owners.v_id?

回答1:

The usual solution to this would be to create an intersection table:

CREATE TABLE vehicles_owned
(
  o_id INT,
  v_id INT,
  PRIMARY KEY (o_id, v_id),
  FOREIGN KEY (o_id)
    REFERENCES owners (o_id),
  FOREIGN KEY (v_id)
    REFERENCES vehicles (v_id)
);

You then drop v_id from owners.

This table allows each owner to own multiple vehicles, and each vehicle to have multiple owners. If you want to enforce a one-owner-per-vehicle constraint, add a UNIQUE index to vehicles_owned.v_id.

EDIT: Of course, if you want to enforce a one-owner-per-vehicle constraint, you could also simply add o_id to vehicles as a foreign key, and not bother with the intersection table.

来源：https://stackoverflow.com/questions/37825685/foreign-key-referring-to-multiple-rows-in-another-table

标签

mysql

foreign-keys