April 2013 Archive


Multiway Relationships

The E/R model makes it easy to describe relationships involving more than two entity sets. In practice, ternary (three-way) or higher-degree relationships are infrequent, but they are rarely necessary to reflect the true state of affairs. A multiway relationship in an E/R diagram

Roles in Relationships

It is possible that one entity set appears two or more times in a single relation ship. If so, we draw as many lines from the relationship to the entity set as the entity set appears in the relationship. Each line to the entity

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

Converting Multiway Relationships to Binary

There are a number of data models, such as ODL (Object Definition Language), which we introduce in Introduction to ODL, that limit relationships to be binary. Therefore, while the E/R model does not require binary relationships,

Subclasses in the E/R Model

Often, an entity set includes certain entities that have special attributes not connected with all members of the set. If so, we find it useful to describe certain special-case entity sets, or subclasses, each with its own special attributes and/or relationships. We join an entity set to

Design Principles

We have yet to learn many of the details of the E/R model, but we have enough to begin study of very important issue of what forms a good design and what should be avoided. In

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

Simplicity Counts

Avoid introducing more elements into your design than is really needed.

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

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