Multiway Relationship

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

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,

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,

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.


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

Additional ODL Concepts

There are many other features of ODL that we must study if we are to express in ODL the things that we can express in the E/R or relational models. In this section, we shall cover:

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