Composite Key

In  certain  tables,  a  one  attribute  cannot  be  used  to  identify  rows  uniquely  and  a combination  of  more  or two attributes  is  used  as  a  primary  key.  That type of keys is known as composite keys.

Let consider the following table of customer that is used to maintain the purchases made through several customers.

Customers

In the table you can see all values are not unique for any of the attributes. Thus, a combination of Productcode and Custcode results in all unique values. Therefore the combination can be used as a composite primary key.