two columns primary key, auto-increment and foreign key - mysql

Question

The following query was successful but when I enter null values into order id and different values(those which do not exist in the column) for item id, the order id is still getting incremented. How to retain the same order id? Is there a problem with the way the foreign key has been stated?
CREATE TABLE orders(
orderid int not null auto_increment,
itemid int not null,
quantity int not null,
tot_price int not null,
cid int not null,
code varchar(10),
order_time time,
order_date date ,
constraint order_pk primary key (orderid, itemid),
foreign key (itemid) references items(itemid),
foreign key (cid) references customer(cid),
foreign key (code) references coupon(code)
);

Answer 1

The problem stems from trying to do a one-to-many (order to items) relationship in a single table. This will end badly. You need two. One for the order, one for the items in the order. For good or bad, this is how relational databases do lists.
CREATE TABLE orders (
orderid int not null primary key auto_increment,
cid int not null references customer(cid),
code varchar(10) references coupon(code),
when_ordered datetime,
INDEX(when_ordered)
);
CREATE TABLE items_in_an_order (
orderid int not null references orders(id),
itemid int not null references items(id),
quantity int not null,
price_paid int not null
);
quantity moves to items_in_an_order. I changed total_price to the more descriptive price_paid which is likely more descriptive of what you want to be storing. This will let you produce receipts.
order_date and order_time were unnecessarily split into two columns. This makes comparison and sorting awkward. Putting them together into one DATETIME column and indexing it will let you do sorting by date/time and check if an order is inside a date/time range.
To get all the items in an order, do a join.
SELECT items.itemid
FROM items
JOIN items_in_an_order io ON io.itemid = items.id
WHERE io.orderid = ?
To find all the orders an item is in, also do a join.
SELECT orders.id
FROM orders
JOIN items_in_an_order io ON io.orderid = orders.id
WHERE io.itemid = ?