Additional Relationship Types

As you develop a logical model, you may find some parent/child relationships that do not fall into the standard, one-to-many relationships. These relationship exceptions include:

Many-to-many relationships

A relationship where one entity <owns> many instances of a second entity, and the second entity also <owns> many instances of the first entity. For example, an EMPLOYEE <has> one or more JOB TITLEs, and a JOB TITLE <is applied to> one or more EMPLOYEEs.

N-ary relationships

A simple one-to-many relationship between two entities is termed binary. When a one-to-many relationship exists between two or more parents and a single child entity, it is termed an n-ary relationship.

Recursive relationships

Entities that have a relationship to themselves take part in recursive relationships. For example, for the EMPLOYEE entity, you could include a relationship to show that one EMPLOYEE <manages> one or more EMPLOYEEs. This type of relationship is also used for bill-of-materials structures, to show relationships between parts.

Subtype relationships

Related entities are grouped together so that all common attributes appear in a single entity, but all attributes that are not in common appear in separate, related entities. For example, the EMPLOYEE entity could be subtyped into FULL-TIME and PART-TIME.