When a user inserts or updates a row, the database server ensures that the primary key for the table is still valid: that each row in the table is uniquely identified by the primary key.
The Employees table in the SQL Anywhere sample database uses an employee ID as the primary key. When you add a new employee to the table, the database server checks that the new employee ID value is unique and is not NULL.
The SalesOrderItems table in the SQL Anywhere sample database uses two columns to define a primary key.
This table holds information about items ordered. One column contains an ID specifying an order, but there may be several items on each order, so this column by itself cannot be a primary key. An additional LineID column identifies which line corresponds to the item. The columns ID and LineID, taken together, specify an item uniquely, and form the primary key.