Question

mysql>  create table products(id integer unsigned auto_increment primary key);
Query OK, 0 rows affected (0.05 sec)

mysql> CREATE TABLE orders (
    ->     id integer PRIMARY KEY auto_increment,
    ->     product_id integer REFERENCES products (id),
    ->     quantity integer,
    ->     INDEX product_id_idx (product_id)
    -> );
Query OK, 0 rows affected (0.05 sec)
mysql> insert into orders(product_id,quantity) value(1,1);
Query OK, 1 row affected (0.00 sec)

Since product 1 doesn’t exist,the insert statement is supposed to fail,but in fact not.

Why?

Answer 1

You should explicitly define the foreign key below the column definitions.

You should also make product_id unsigned since the parent key is unsigned:

CREATE TABLE orders (
  id integer PRIMARY KEY auto_increment,
  product_id integer unsigned,
  quantity integer,
  INDEX product_id_idx (product_id),
  CONSTRAINT FK_ORDER_TO_PRODUCT FOREIGN KEY (product_id) REFERENCES products (id)
 ) engine=innodb;