Enhanced Entity-Relationship Model (EER) 1

Enhanced-ER (EER) Model Concepts Includes all modeling concepts of basic ER Additional concepts: subclasses/superclasses, specialization/generalization, categories, attribute inheritance The resulting model is called the enhanced-ER or Extended ER (E2R or EER) model It is used to model applications more completely and accurately if needed It includes some object-oriented concepts, such as inheritance 2

Enhanced Entity-Relationship Model (EER) 3 EER is an ER model supported with additional semantic concepts. Semantic concepts supported: - Specialization - Generalization - Aggregation

Specialization 4 Top-down design process; we designate sub-groupings within an entity type that are distinctive from other entities in the set. These sub-groupings (subclasses) become lower-level entity types that have attributes or participate in relationships that do not apply to the higher-level entity set (superclass).

Specialization/Generalization 5 STAFF MANAGERSECRETARY SALES PERSONNEL Subclass Superclass IS_A relationship 1:1

Generalization 6 A bottom-up design process – combine a number of entity types that share the same features into a higher-level (superclass) entity type. Specialization and generalization are simple inversions of each other; they are represented in an EER diagram in the same way.

Inheritance 7 A subclass entity type inherits all the attributes and relationship participation of the superclass entity type to which it is linked.

Inheritance 8 STAFF SALES PERSONNEL Unshared attributes Superclass addressDOBname Sales area Car allowance shared attributes Subclass require CAR Unshared relationship contract COMPANY Shared relationship

Subclasses and Superclasses (1) An entity type may have additional meaningful subgroupings of its entities Example: EMPLOYEE may be further grouped into SECRETARY, ENGINEER, MANAGER, TECHNICIAN, SALARIED_EMPLOYEE, HOURLY_EMPLOYEE,… Each of these groupings is a subset of EMPLOYEE entities Each is called a subclass of EMPLOYEE EMPLOYEE is the superclass for each of these subclasses These are called superclass/subclass relationships. Example: EMPLOYEE/SECRETARY, EMPLOYEE/TECHNICIAN 9

Subclasses and Superclasses (2) These are also called IS-A relationships (SECRETARY IS-A EMPLOYEE, TECHNICIAN IS-A EMPLOYEE, …). Note: An entity that is member of a subclass represents the same realworld entity as some member of the superclass – The Subclass member is the same entity in a distinct specific role – An entity cannot exist in the database merely by being a member of a subclass; it must also be a member of the superclass – A member of the superclass can be optionally included as a member of any number of its subclasses Example: A salaried employee who is also an engineer belongs to the two subclasses ENGINEER and SALARIED_EMPLOYEE – It is not necessary that every entity in a superclass be a member of some subclass 10

Constraints on Specialization/Generalization 11 Participation constraint determines whether every member in the superclass must participate as a member of a subclass. Two types of participation constraints: - Mandatory (total) - Optional (partial)

Attribute Inheritance in Superclass / Subclass Relationships An entity that is member of a subclass inherits all attributes of the entity as a member of the superclass It also inherits all relationships 12

Participation Constraints 13 Mandatory (total) participation where every member in the superclass must also be a member of a subclass. STAFF FULL-TIME STAFF PART-TIME STAFF salary Hourly-rate

Participation Constraints 14 Optional (partial) participation where a member in the superclass need not belong to any of its subclasses. STAFF MANAGERSECRETARY SALES PERSONNEL

Constraints on Specialization/Generalization 15 Disjoint constraint describes the relationship between members of the subclasses & indicates whether it is possible for a member of a subclass to be a member of one or more subclasses. Two types of disjoint constraints: - Disjoint - Non-Disjoint

Disjoint Constraints 16 Disjoint constraint when an entity can be a member of only one of the subclasses of the specialization. STAFF FULL-TIME STAFF PART-TIME STAFF salary Hourly-rate d

Disjoint Constraints 17 Non-disjoint constraints: a subclass is a member of more than one subclass of specialization. Entity types may overlap. STAFF MANAGERSECRETARY SALES PERSONNEL o

Specialization Again Is the process of defining a set of subclasses of a superclass The set of subclasses is based upon some distinguishing characteristics of the entities in the superclass Example: {SECRETARY, ENGINEER, TECHNICIAN} is a specialization of EMPLOYEE based upon job type. – May have several specializations of the same superclass Example: Another specialization of EMPLOYEE based in method of pay is {SALARIED_EMPLOYEE, HOURLY_EMPLOYEE}. – Superclass/subclass relationships and specialization can be diagrammatically represented in EER diagrams – Attributes of a subclass are called specific attributes. For example,TypingSpeed of SECRETARY – The subclass can participate in specific relationship types. For example, BELONGS_TO of HOURLY_EMPLOYEE 18

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Generalization Again 20 The reverse of the specialization process Several classes with common features are generalized into a superclass; original classes become its subclasses Example: CAR, TRUCK generalized into VEHICLE; both CAR, TRUCK become subclasses of the superclass VEHICLE. – We can view {CAR, TRUCK} as a specialization of VEHICLE – Alternatively, we can view VEHICLE as a generalization of CAR and TRUCK

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Aggregation 22 Represents a “part-of” relationship between entity types, where one represents the ‘whole’ and the other the ‘part’. No inherited attributes; each entity has its own unique set of attributes.

Aggregation 23 TEAM MEMBER

Summary of ERD notations (1) 24 ENTITY WEAK ENTITY RELATIONSHIP IDENTIFYING RELATIONSHIP ATTRIBUTE KEY ATTRIBUTE MULTI-VALUED COMPOSITE DERIVED

Summary of ERD notations (2) 25 CARDINALITY RATION PARTICIPATION CONSTRAINTS 1 M (min,max)

Summary of EERD notations (3) 26 Disjoint constraint d o Non-Disjoint constraint Total Participation Optional Participation