A relationship is an association between entities (Chen, 1976). A relationship captures the high-level interactions between conceptual entities, the detailed interactions between logical entities, and the constraints between physical entities.
The term relationship can go by other names. Relationship aliases can vary based on scheme. In relational schemes the term relationship is often used, dimensional schemes the term navigation path is often used, and in NoSQL schemes terms such as edge or link are used, for example. Relationship aliases can also vary based on level of detail. A relationship at the conceptual and logical levels is called a relationship, but a relationship at the physical level may be called by other names, such as constraint or reference, depending upon the database technology.
Graphic Representation of Relationships
Relationships are shown as lines on the data modeling diagram. This figure is an Information Engineering example.
In this example, the relationship between Student and Course captures the rule that a Student may attend Courses. The relationship between Instructor and Course captures the rule that an Instructor may teach Courses. The symbols on the line (called cardinality) capture the rules in a precise syntax. (These will be explained next week). A relationship is represented through foreign keys in a relational database and through alternative methods for NoSQL databases such as though edges or links.
DAMA Data Management Body of Knowledge 2nd Edition, 2017, Print.