How to refer to a composite key?

Question

MySQL

Good afternoon experts.
When learning SQL, I asked myself a question, but could not find an answer (oddly enough...):
we have 3 tables: users, professions, user_professions (for example). There is a many-to-many relationship between users and professions through the user_professions table. Accordingly, user_professions has a composite PRIMARY KEY in the form (users_id, profession_id). But there are situations when the connecting table is also a separate entity and someone will have to refer to it through FOREIGHN KEY. But how then to refer to the composite key?

Answer 1

dev.mysql.com/doc/refman/5.7/en/create-table-forei...

CREATE TABLE product (
    category INT NOT NULL, id INT NOT NULL,
    price DECIMAL,
    PRIMARY KEY(category, id)
)   ENGINE=INNODB;

CREATE TABLE customer (
    id INT NOT NULL,
    PRIMARY KEY (id)
)   ENGINE=INNODB;

CREATE TABLE product_order (
    no INT NOT NULL AUTO_INCREMENT,
    product_category INT NOT NULL,
    product_id INT NOT NULL,
    customer_id INT NOT NULL,

    PRIMARY KEY(no),
    INDEX (product_category, product_id),
    INDEX (customer_id),

    FOREIGN KEY (product_category, product_id)
      REFERENCES product(category, id)
      ON UPDATE CASCADE ON DELETE RESTRICT,

    FOREIGN KEY (customer_id)
      REFERENCES customer(id)
)   ENGINE=INNODB;

Answer 2

And why is there a problem with a reference to a composite key? :) The mechanism is exactly the same.