/* Primary Key Creation using Alter */
Use Demo_Database

Alter Table Customer With Nocheck
     Add Constraint PK_Customer Primary Key Clustered
     (Customer_ID) 
     On [Primary]
Go

Alter Table Order_Invoice With Nocheck
     Add Constraint PK_Order_Invoice Primary Key Clustered
     (Order_ID) 
     On [Primary]
Go

Alter Table Order_Invoice_Items With Nocheck
     Add Constraint PK_Order_Invoice_Items Primary Key Clustered
     (Order_ID, Item_ID) 
     On [Primary]
Go

Alter Table Order_Invoice With Check
     Add Constraint FK_Customer_ID
     Foreign Key
     (Customer_ID) 
     References Customer (Customer_ID)
Go