When a primary key is created from a combination of
2 or more columns, the primary key is called a composite
key. Each column may not be unique by itself within
the database table but when combined with the other
column(s) in the composite key, the combination is unique.
To illustrate the concept of the composite key and the
foreign key, consider the sample table design below:
The lastname column and the firstname column together
form a composite key. Let's assume that the above tables
contain the following data:
lastname firstname
dateofbirth
-------- --------- -----------
henry john 03/05/1960
henry adam 06/08/1974
kidman adam 04/01/1955
bailey harry 05/05/1980
morgan alex 09/09/1975
Notice that in the lastname column, there are 2 records
with the value 'henry' and in the firstname column,
there are 2 records with the value 'adam'. However,
there are no records in the database table with a duplicate
combination of both the lastname and the firstname.
