Question

I have a question about sql relation conception. I have a simple example.
A table person, country and I want to record the history where person had lived.

CREATE TABLE person
(
    person_id INT UNSIGNED PRIMARY KEY AUTO_INCREMENT
);

CREATE TABLE country
(
    country_id INT UNSIGNED PRIMARY KEY AUTO_INCREMENT
);

First solution :

create table person_live_country
(
    plc_id INT UNSIGNED PRIMARY KEY AUTO_INCREMENT,
    person_id INT UNSIGNED NOT NULL,
    country_id INT UNSIGNED NOT NULL,
    FOREIGN KEY (person_id) REFERENCES person (person_id),
    FOREIGN KEY (country_id) REFERENCES country (country_id)
);

Second solution :

create table person_live_country
(
    person_id INT UNSIGNED NOT NULL,
    country_id INT UNSIGNED NOT NULL,
    PRIMARY KEY (person_id, country_id),
    FOREIGN KEY (person_id) REFERENCES person (person_id),
    FOREIGN KEY (country_id) REFERENCES country (country_id)
);

What is the best pattern : conception, performance, convenience ? In the future I want to map the tables with JPA.

Thanks.

Answer 1

While the second solution is sufficient and slightly space efficient, I would recommend the first solution for the following reasons:

• If you want to extend your table design in future, for example to add fromdate and todate cloumns in person_live-country table, the composit primary key in the second solution will no longer work. The same person can live in the same country more than once in his lifetime.
• If you need to refer to person_live-country table from another table as a foreign key, the fisrt solution is easier.