sql 中一个表设置两个主键是 将两个字段联合起来设置为主键 一个表只能有一个主键

CREATE TABLE orderitems (
order_num int(11) NOT NULL,
order_item int(11) NOT NULL,
prod_id char(10) NOT NULL,
quantity int(11) NOT NULL,
item_price decimal(8,2) NOT NULL,
PRIMARY KEY (order_num,order_item),
KEY FK_OrderItems_Products (prod_id),
CONSTRAINT FK_OrderItems_Orders FOREIGN KEY (order_num) REFERENCES orders (order_num),
CONSTRAINT FK_OrderItems_Products FOREIGN KEY (prod_id) REFERENCES products (prod_id)
) ENGINE=InnoDB DEFAULT CHARSET=utf8;