11. Types of primary keys: Compound

There are two types of primary keys:

  1. Simple
  2. Compound or composite

Compound or composite primary key

DEFINITION:

A compound / composite primary key combines more than one field to make a unique value.

 

Example 1compound primary key

Sometimes, a single field is not enough to uniquely identify a record.

Consider a pack of cards. There are four suits, hearts, diamonds, clubs and spades. A single primary key would not uniquely identify a particular card based just upon the suit.

There are also thirteen values from the Ace right through to the King. Again, this would not be sufficient to uniquely identify a card.

But, if you made a primary key out of the suit and the value, then every card in the pack could be uniquely identified.

 

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Example 2compound primary key

A student can only be in one place at a time.

Combining the student name, date and period gives a unique value for each record.

The three fields: student, date and period make up the compound or composite primary key in this table.