Composite key in DBMS

Definition of Composite key: A key that has more than one attributes is known as composite key. It is also known as compound key.

Note: Any key such as super key, primary key, candidate key etc. can be called composite key if it has more than one attributes.

Composite key Example

Lets consider a table Sales. This table has four columns (attributes) – cust_Id, order_Id, product_code & product_count.

Table – Sales

cust_Id    order_Id    product_code    product_count
--------   --------    ------------    -------------
C01	   O001         P007           23
C02	   O123	        P007           19
C02	   O123	        P230           82
C01	   O001	        P890           42

None of these columns alone can play a role of key in this table.

Column cust_Id alone cannot become a key as a same customer can place multiple orders, thus the same customer can have multiple entires.

Column order_Id alone cannot be a primary key as a same order can contain the order of multiple products, thus same order_Id can be present multiple times.

Column product_code cannot be a primary key as more than one customers can place order for the same product.

Column product_count alone cannot be a primary key because two orders can be placed for the same product count.

Based on this, it is safe to assume that the key should be having more than one attributes:
Key in above table: {cust_id, product_code}

This is a composite key as it is made up of more than one attributes.