dbms keys: comparing compound and composite keys



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What is the Compound key?

COMPOUND KEY has two or more attributes that allow you to uniquely recognize a specific record. It is possible that each column may not be unique by itself within the database. However, when combined with the other column or columns the combination of composite keys become unique. The purpose of the compound key in database is to uniquely identify each record in the table.


OrderNo PorductID Product Name Quantity
B005 JAP102459 Mouse 5
B005 DKT321573 USB 10
B005 OMG446789 LCD Monitor 20
B004 DKT321573 USB 15
B002 OMG446789 Laser Printer 3

In this example, OrderNo and ProductID can’t be a primary key as it does not uniquely identify a record. However, a compound key of Order ID and Product ID could be used as it uniquely identified each record.

What is the Composite key?

COMPOSITE KEY is a combination of two or more columns that uniquely identify rows in a table. The combination of columns guarantees uniqueness, though individually uniqueness is not guaranteed. Hence, they are combined to uniquely identify records in a table.

The difference between compound and the composite key is that any part of the compound key can be a foreign key, but the composite key may or maybe not a part of the foreign key.

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