Identifying Entities and Relationships › Identifying Relationships Among Entities › Types of Data Relationships

Types of Data Relationships

Data entities in a database are related in one of three ways: one-to-one (1-1), one-to-many (1-M), and many-to-many (M-M). Each of these types of relationships is explained below.

One-to-one (1-1)

In the one-to-one example below, for every EMPLOYEE entity occurrence in the database, there can exist only one corresponding PROJECT entity occurrence.

One-to-many (1-M)

In the one-to-many example below, for every DEPARTMENT entity occurrence in the database, there may exist one or more corresponding EMPLOYEE entity occurrences.

Many-to-many (M-M)

In the many-to-many example below, for every SKILL entity occurrence in the database, there can exist one or more corresponding EMPLOYEE entity occurrences; for every EMPLOYEE entity occurrence in the database, there can also exist one or more corresponding SKILL entity occurrences.

Other types of data relationships

In addition to relationships between two entity types, the following types of data relationships are acceptable in an entity-relationship model:

• A relationship can be defined for only one entity type. For example, to define a relationship between different employees in an organization, you might want to combine different data occurrences from the EMPLOYEE entity. In this case, the relationships among employees might be expressed as MANAGES and REPORTS TO, as shown in the entity-relationship diagram of Commonweather Corporation.