MySQLでN：Mの関係を持つテーブルを作成するにはどうすればよいですか？

多対1（製品は1つのストアしか持てません）

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)

多対多（製品は多くの店舗にある可能性があります）

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)