Every table in the asiqdemo database has a primary key. (It is a good idea to have a primary key for each table.) 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—emp_id is the primary key of the employee table.
The sales_order_items 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 sales_order_items table because there can be several items in an order. Also, the line_id number does not uniquely identify a row in the sales_order_items table. Both the order ID name and line_id are required to uniquely identify a row in the sales_order_items table. The primary key of the table is both columns taken together.
