Key

Representing Keys in the E/R Model

In our E/R diagram notation, we underline the attributes belonging to a key for an entity set. For instance, the following figure reproduces our E/R diagram for movies, stars, and studios

Weak Entity Sets

There is a special condition in which an entity set's key is consisting of attributes some or all of which belong to another entity set. Such an entity set is called a weak entity set.

Summary of The Entity-Relationship Data Model

The Entity/Relationship Model: In the E/R model we explain entity sets, relationships among entity sets, and attributes of entity sets and relationships. Members of entity sets are called entities.

Keys of Relations

We say a set of one or more attributes {A1, A2,. . . ,An} is a key for a relation R if: 1. Those attributes functionally determine all other attributes of the relation. That is, because relations are sets, it is impossible for two different tuples of R to agree on all of A1, A2, ... ,An.

Superkeys

A set of attributes that includes a key is called a superkey, short for "superset of a key". In this way, every key is a superkey. However, some superkeys are not (minimal) keys. Note

Fourth Normal Form

The redundancy that we found in "Multivalued Dependencies" to be caused by MVD's can be removed if we use these dependencies in a new decomposition algorithm for relations. In this section we shall introduce a new normal form, called "fourth normal form". In this normal form,

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