与三个不同表相关的三个主键

Question

I have the following tables category, Restaurant, and menu.

A restaurant can have more than one menu, and a restaurant can also have more than one category. Each menu obviously has some number of catagories, and each catagory can be a part of a number of menus.

Can I create a table that looks like this, and has 3 primary keys?

Category ID | Restaurant ID | Menu ID

1 (Appetizer) | 1 Chile's | 2 Dinner Menu
2 (Entree) | 1 Chile's | 2 Dinner Menu
3 (Dessert) | 1Chile's | 2 Dinner Menu
1 | 67 McDonalds | 3 Lunch Menu 1

Answer 1

Yes, you can make a primary key of the tuple (category_id, restaurant_id, menu_type_id) . Better even, make a normal, integral primary key, and add another index on that tuple with uniqueness constraint. It's always good to have a fast, simple primary key, but you can certainly enforce uniqueness of the triple:

CREATE UNIQUE INDEX menu_index
                 ON menus (category_id, restaurant_id, menu_type_id)

Or you can define the index right in the table creation:

CREATE TABLE menus (menu_id       INT AUTO_INCREMENT PRIMARY KEY,
                    category_id   INT,
                    restaurant_id INT,
                    menu_type_id  INT,
                    FOREIGN KEY (category_id)   REFERENCES categories(category_id),
                    FOREIGN KEY (restaurant_id) REFERENCES restaurants(restaurant_id),
                    FOREIGN KEY (menu_type_id)  REFERENCES menu_types(menu_type_id),
                    UNIQUE INDEX  (category_id, restaurant_id, menu_type_id)
                   ) ENGINE=InnoDB;