How to create tables with N:M relationship in MySQL?

Question

Let\'s say I have this two tables: Store and Product. I want my store to have a list of products. How can I do that?

create table store(
id int unsigned not          


        

Answer 1

Many-to-one (products can only have one store)

create table store(
    id int unsigned not null auto_increment,
    store_name varchar(30) not null,
    primary key(id)
);

Query OK, 0 rows affected (0.02 sec)

create table product(
    id int unsigned not null auto_increment,
    store_id int unsigned not null,
    product_name varchar(30) not null,
    price float not null,
    primary key(id),
    constraint product_store foreign key (store_id) references store(id)
);

Query OK, 0 rows affected (0.02 sec)

Many-to-many (products can be in many stores)

create table store(
    id int unsigned not null auto_increment,
    store_name varchar(30) not null,
    primary key(id)
);

Query OK, 0 rows affected (0.04 sec)

create table product(
    id int unsigned not null auto_increment,
    store_id int unsigned not null,
    product_name varchar(30) not null,
    price float not null,
    primary key(id)
);

Query OK, 0 rows affected (0.01 sec)

create table product_store (
    product_id int unsigned not null,
    store_id int unsigned not null,
    CONSTRAINT product_store_store foreign key (store_id) references store(id),
    CONSTRAINT product_store_product foreign key (product_id) references product(id),
    CONSTRAINT product_store_unique UNIQUE (product_id, store_id)
)

Query OK, 0 rows affected (0.02 sec)

Answer 2

This is an n-m relationship. One store has multiple products. One product is in multiple stores (presumably).

You do this with a separate table:

create table StoreProducts (
    StoreProductId int auto_increment primary key,
    StoreId int,
    ProductId int,
    constraint fk_storeproducts_store foreign key (StoreId) references Stores(StoreId),
    constraint fk_storeproducts_product foreign key (ProductId) references Products(ProductId)
);

This is called a junction table. You can maintain additional information in the table, such as a "no longer stocked" flag or "date of first stock".