sql - Many-to-many relations in RDBMS databases

Question

What is the best way of handling many-to-many relations in a RDBMS database like mySQL?

Have tried using a pivot table to keep track of the relationships, but it leads to either one of the following:

• Normalization gets left behind

• Columns that is empty or null

What approach have you taken in order to support many-to-many relationships?

Answer 1

Keep track of a many-to-many relationship in a table specifically for that relationship (sometimes called a junction table). This table models the relationship as two one-to-many relationships pointing in opposite directions.

CREATE TABLE customer (
    customer_id VARCHAR NOT NULL,
    name VARCHAR NOT NULL,
    PRIMARY KEY (customer_id));

CREATE TABLE publication (
    issn VARCHAR NOT NULL,
    name VARCHAR NOT NULL,
    PRIMARY KEY (issn));

-- Many-to-many relationship for subscriptions.
CREATE TABLE subscription (
    customer_id VARCHAR NOT NULL,
        FOREIGN KEY customer_id REFERENCES customer (customer_id),
    issn VARCHAR NOT NULL,
        FOREIGN KEY issn REFERENCES publication (issn),
    begin TIMESTAMP NOT NULL,
    PRIMARY KEY (customer_id, issn));

You then use the junction table to join other tables through it via the foreign keys.

-- Which customers subscribe to publications named 'Your Garden Gnome'?
SELECT customer.*
FROM customer
    JOIN subscription
        ON subscription.customer_id = customer.customer_id
    JOIN publication
        ON subscription.issn = publication.issn
WHERE
    publication.name = 'Your Garden Gnome';

-- Which publications do customers named 'Fred Nurk' subscribe to?
SELECT publication.*
FROM publication
    JOIN subscription
        ON subscription.issn = publication.issn
    JOIN customer
        ON subscription.customer_id = customer.customer_id
WHERE
    customer.name = 'Fred Nurk';