Keys

Differences between entities must be expressed in terms of attributes.

• A superkey is a set of one or more attributes which, taken collectively, allow us to identify uniquely an entity in the entity set.

• For example, in the entity set customer, customer-name and S.I.N. is a superkey.

• Note that customer-name alone is not, as two customers could have the same name.

• A superkey may contain extraneous attributes, and we are often interested in the smallest superkey. A superkey for which no subset is a superkey is called a candidate key.

• In the example above, S.I.N. is a candidate key, as it is minimal, and uniquely identifies a customer entity.

• A primary key is a candidate key (there may be more than one) chosen by the DB designer to identify entities in an entity set.

For example,

• The entity set transaction has attributes transaction-number, date and amount.
• Different transactions on different accounts could share the same number.
• These are not sufficient to form a primary key (uniquely identify a transaction).
• Thus transaction is a weak entity set.

For a weak entity set to be meaningful, it must be part of a one-to-many relationship set. This relationship set should have no descriptive attributes. (Why?)

The idea of strong and weak entity sets is related to the existence dependencies seen earlier.

• Member of a strong entity set is a dominant entity.
• Member of a weak entity set is a subordinate entity.

A weak entity set does not have a primary key, but we need a means of distinguishing among the entities.

The discriminator of a weak entity set is a set of attributes that allows this distinction to be made.

The primary key of a weak entity set is formed by taking the primary key of the strong entity set on which its existence depends (see Mapping Constraints) plus its discriminator.

To illustrate:

• transaction is a weak entity. It is existence-dependent on account.
• The primary key of account is account-number.
• transaction-number distinguishes transaction entities within the same account (and is thus the discriminator).
• So the primary key for transaction would be (account-number, transaction-number).

Just Remember: The primary key of a weak entity is found by taking the primary key of the strong entity on which it is existence-dependent, plus the discriminator of the weak entity set.