Designing a rudimentary Shopping Cart database - sql

Question

create table [User]
(
UserId int primary key identity(1,1),
FirstName nvarchar(256) not null,
LastName nvarchar(256) not null,
)
create table Product
(
ProductId int primary key identity(1,1),
UnitPrice decimal(18,2) not null, //For catalog purposes.
Name nvarchar(1000) not null,
Description nvarchar(max) not null,
Stock int not null
)
create table [Order]
(
OrderId int primary key identity(1,1),
UserId int foreign key references [User](UserId),
ProductId int foreign key references Product(ProductId),
UnitCost decimal(18,2) not null, //How much it actually cost when the person bought it.
ItemCount int not null,
Subtotal decimal(18,2) not null
)
create table OrderDetail
(
OrderDetailId int primary key identity(1,1),
?
I'm stuck on the database design of the order system.
A user can choose n products to add to a order request. Any suggestions?
Following some advice given here, how would this feel? Any pitfalls?
create table [User]
(
UserId int primary key identity(1,1),
FirstName nvarchar(256) not null,
LastName nvarchar(256) not null,
)
create table Product
(
ProductId int primary key identity(1,1),
UnitPrice decimal(18,2) not null,
Name nvarchar(1000) not null,
Description nvarchar(max) not null,
Stock int not null
)
create table [Order]
(
OrderId int primary key identity(1,1),
UserId int foreign key references [User](UserId),
DateOfOrder datetime not null
)
create table OrderDetail
(
OrderDetailId int primary key identity(1,1),
OrderId int foreign key references [Order](OrderId),
ProductId int foreign key references Product(ProductId),
UnitCost decimal(18,2) not null,
ItemCount int not null,
Subtotal decimal(18,2) not null
)

Answer 1

Typically, you'd have the Order table with the top-level order information (who, when etc) and then an OrderItem (or OrderDetail) table which has a row for each product that forms part of the order including columns like:
OrderId
ProductId
Quantity
etc
Good candidate for a PK on this OrderItem/OrderDetail table would be on OrderId + ProductId.
So where you have columns like ProductId, UnitCost, ItemCount etc in the Order table, those are in the wrong place and should be in the OrderItem/OrderDetail table.
Update:
To set up a compound PK, you can do:
create table OrderDetail
(
OrderId int foreign key references [Order](OrderId),
ProductId int foreign key references Product(ProductId),
...other columns...,
CONSTRAINT PK_OrderDetail PRIMARY KEY(OrderId, ProductId)
)