create table orders(
                     id               int                        not null                 auto_increment,
                     name          varchar(100)          not null,
primary key (id)
);create table line_items
                   id                  int                        not null                 auto_increment,
                   order_id         int                        not null,
                   constraint       fk_items_order      foreign   key (order_id)                      references orders(id),
primary key (id)
):这个外键引用时 fk_items_order 是要遵从一定的格式的么? 
有一定规定的么?