Sometimes more than one attribute needs to create a key that uniquely represents a tuple. That keys are known as a composite key or compound key.

Any>super key,>primary key,>candidate key>can be called a composite key if it has more than one attribute.

Example

Lets consider a table Sales with four attributes or columns – customer_id, order_id, pcode and qty.

Here, none of these columns can declare as a primary key that uniquely identifies a tuple. Because

1. One customer can place multiple orders, so customer_id>alone cannot become a key.
2. One order may contain multiple products, So order_id>alone cannot become a primary key
3. More than one customer can order the same product, So pcode>alone cannot become a primary key.
4. More than one order can be placed with the same qty value, so Qty>alone cannot become a primary key.

Here only the combination of customer_id and order_id can uniquely identify a tuple and can be declared as the primary key. That is why the said set of keys known as a composite key.