Database Modeling & Design Fourth Edition- P7: Database technology has evolved rapidly in the three decades since the rise and eventual dominance of relational database systems. While many specialized database systems (object-oriented, spatial, multimedia, etc.) have found substantial user communities in the science and engineering fields, relational systems remain the dominant database technology for business enterprises. | Fundamental ER Constructs 17 Concept Degree recursive binary binary ternary Connectivity one-to-one one-to-many many-to-many Existence optional mandatory Representation Example Figure Degrees connectivity and attributes of a relationship 18 CHAPTER 2 The Entity-Relationship Model Connectivity of a Relationship The connectivity of a relationship describes a constraint on the connection of the associated entity occurrences in the relationship. Values for connectivity are either one or many. For a relationship between the entities Department and Employee a connectivity of one for Department and many for Employee means that there is at most one entity occurrence of Department associated with many occurrences of Employee. The actual count of elements associated with the connectivity is called the cardinality of the relationship connectivity it is used much less frequently than the connectivity constraint because the actual values are usually variable across instances of relationships. Note that there are no standard terms for the connectivity concept so the reader is admonished to consider the definition of these terms carefully when using a particular database design methodology. Figure shows the basic constructs for connectivity for binary relationships one-to-one one-to-many and many-to-many. On the one side the number one is shown on the connection between the relationship and one of the entities and on the many side the letter N is used on the connection between the relationship and the entity to designate the concept of many. In the one-to-one case the entity Department is managed by exactly one Employee and each Employee manages exactly one Department. Therefore the minimum and maximum connectivities on the is-man-aged-by relationship are exactly one for both Department and Employee. In the one-to-many case the entity Department is associated with has many Employees. The maximum connectivity is given on the Employee many side as the unknown .
