Relationship

Elements of the E/R Model

The most common model for abstract representation of the structure of a database is the entity-relationship model (or E/R model). In the E/R model, the structure of data is represented graphically, as an entity-relationship diagram," using three principal element

Instances of an E/R Diagram

E/R diagrams provide details for describing the schema of databases, that is, their structure. A database described by an E/R diagram will include specific data, which we call the database instance. Particularly, for each entity set, the database instance will have a particular finite

Attributes on Relationships

Occasionally it is convenient, or even necessary, to connect attributes with a relationship, rather than with any one of the entity sets that the relationship connects. For example, consider the relationship of Multiway Relationships figure, which represents contracts

Avoiding Redundancy

We should be careful to say everything once only. For example, we have used a relationship Owns between movies and studios. We might also choose to have an attribute studioName of

Choosing the Right Relationships

Entity sets can be joined in different ways by relationships. However, adding to our design every possible relationship is not often a good idea. First, it can lead to redundancy, where

Picking the Right Kind of Element

Sometimes we have options about the type of design element used to represent a real-world conception. Many of these choices are between using attributes and using entity

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

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

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.

From E/R Relationships to Relations

Relationships in the E/R model are also represented by relations. The relation for a given relationship R has the following attributes:

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