303 Cardinality and Participation PDF

303 Cardinality and Participation In the ER model, ** Degree of Relationship, ** ** Cardinality and **...

303 Cardinality and Participation In the ER model, ** Degree of Relationship, ** ** Cardinality and ** ** Participation ** are essential concepts that describe the constraints on relationships between entities. == *** ** *== These concepts specify how many instances of one entity can or must be associated with * == instances of another entity , helping to clarify the nature of interactions between data == * * * * * elements. Degree of Relationship The Degree Of A Relationship in an ER model indicates the number of entity types involved **~ ~** * == in a relationship. It provides information on the complexity of interactions between entities. == * * * Unary (Degree 1) A Unary Relationship (also known as a Recursive Relationship) involves only one entity **~ ~** *** *** * == type. == * An entity relates to itself, meaning instances of the same entity (type) are associated with * * * == each other. == * ** Example: In an Employee entity (type), an employee (instance) can supervise another ** ` ` * * ` ` * * * * ` employee (instance), creating a supervisor-subordinate relationship within the same entity ` * * * * * (type). * Binary (Degree 2) A Binary Relationship involves two entity types. **~ ~** ==* ** ** *== Binary relationships are the most common and typically represent associations between * * different entities. ** Example: A Student enrolling in a Course is a binary relationship between Student and ** ` ` ` ` * ` ` ` Course entities. ` * Ternary (Degree 3) A Ternary Relationship can occur in a scenario where an Employee works in a specific ~** **~ * ` ` ` Department at a particular Location. The interaction among these three entities cannot ` ` ` * be accurately represented using only binary relationships, as the information about the department and location for an employee would be lost. Entities: ** ` Employee : Represents the individuals working in the organization. ` ** ** ` Department : Represents the functional units of the organization. ` ** ** ` Location : Represents the geographical locations where the organization operates. ` ** * Ternary Relationship: "Works In": * * * This relationship connects the Employee , Department , and Location. ` ` ` ` ` ` It captures the specific connection between an employee working in a department at a particular location. (Also see 303 Appendix: “Improving ER Diagram Accuracy: Representing Employee, [[ Department, and Location Relationships with Binary Associations”) ]] ** Cardinality ** ** Cardinality defines the maximum number of instances of one entity that can be associated ** * ==** ** with a single instance of another entity. == * ** Cardinality constraints include: ** * * ** One-to-One (1:1) ** In a One-To-One Relationship, each instance of Entity A is associated with only one **~ ~** instance of Entity B, and vice versa. ** Example: Each Instructor may have one Office assigned to them, and each Office can ** ` ` ` ` ` ` only belong to one Instructor. ` ` ** One-to-Many (1:N) ** In a One-To-Many Relationship, **~ ~** * each instance of Entity A can relate to multiple instances of Entity B, ** ** * *** but each instance of Entity B relates to only one instance of Entity A. ** ** ** * ** Example: A Department can offer multiple Courses , but each Course is associated with ** ** ` ` ** ** ` ` ** *** *** ** ` ` ** only one Department. ** ` ` ** ** Many-to-One (N:1) ** In a Many-To-One Relationship, **~ ~** * multiple instances of Entity A can be associated with a single instance of Entity B , ` ` ` ` * *** but each instance of Entity A relates to only one instance of Entity B. ** ` ` ` ` * ** Example: ** *** Multiple Students may be advised by one Instructor , *** ` ` *** *** ` ` *** but each Student has only one assigned Instructor. *** ` ` *** *** * ` ` * ** Many-to-Many (M:N) ** In a Many-To-Many Relationship, instances of Entity A can relate to multiple instances of **~ ~** ` `  ` Entity B , and vice versa. ` ** Example: ** ` Students can enroll in multiple Courses , and ` *** *** ` ` each Course can have multiple Students enrolled. ` ` *** *** ` ` ** Participation ** ==** Participation defines whether all or only some instances of an entity are involved in a ** * ** ** relationship. *== There are two types of participation: * * ** ** * Total Participation * In Total Participation, every instance of an entity must participate in the relationship. **~ ~** *** ** * == This is indicated with a double line in ER diagrams. *** *** == ** Example: ** If every Student must enroll in at least one Course , then Student has total participation * * ** ` ` ** * * ** ` ` ** ** ` ` ** * ** ** ** ** in the "enrolls in" relationship. * * Partial Participation * In Partial Participation, only some instances of an entity participate in the relationship. **~ ~** *** ** * == This is represented by a single line in ER diagrams. *** *** == ** Example: If some Instructors do not have a dedicated Office, then Instructor has partial ** ** ** ** ** ** ** ** participation in the "assigned to" relationship with Office. ** ** **