The Entity-Relationship Data Model

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.

Weak Entity Set Notation

We shall adopt the following principles to show that an entity set is weak and to declare its key attributes.

Requirements for Weak Entity Sets

We cannot get key attributes for a weak entity set indiscriminately. Rather, if E is a weak entity set then its key comprises:

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.

Other Kinds of Constraints

As already mentioned at the beginning of this section, there are other kinds of constraints one could wish to enforce in a database. We shall only touch briefly on these here, with the

Referential Integrity in E/R Diagrams

We can extend the arrow notation in E/R diagrams to show whether a relationship is expected to support referential integrity in one or more directions. Suppose R is a relationship from

Referential Integrity

While single-value constraints show that at most one value exists in a given role, a referential integrity constraint asserts that exactly one value exists in that role. We could see

Single-Value Constraints

Sometimes, an important property of a database design is that there is at most one value playing a specific role. For instance, we assume that a movie entity has a unique title, year,

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

Keys in the E/R Model

A key for an entity set E is a set K of one or more attributes such that, given any two separate entities e1 and e2 in E, e1 and e2 cannot have the same values for each of the

Page 0 of 3 Previous 1 2 3 Next