Every table in the iqdemo database has a primary key. A primary key is one or more columns that uniquely identify a row in the table. For example, an employee number uniquely identifies an employee—EmployeeID is the primary key of the Employees table.
The SalesOrderItems table is an example of a table with two columns that make up the primary key. The order ID by itself does not uniquely identify a row in the SalesOrderItems table because there can be several items in an order. Also, the LineID number does not uniquely identify a row in the SalesOrderItems table. Both the order ID name and LineID are required to uniquely identify a row in the SalesOrderItems table. Therefore, the primary key of the table is both columns taken together.
