Chapter 4: The Enhanced ER Model and Business Rules Modern Database Management 8th Edition Jeffrey A. Hoffer, Mary B. Prescott, Fred R. McFadden By: Aatif Kamal Dated: March 2008

Objectives Definition of terms Use of supertype/subtype relationships Use of generalization and specialization techniques Specification of completeness and disjointness constraints Develop supertype/subtype hierarchies for realistic business situations Develop entity clusters Explain universal data model Name categories of business rules Define operational constraints graphically and in English

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

Supertypes and Subtypes Subtype: A subgrouping of the entities in an entity type that has attributes distinct from those in other subgroupings Supertype: A generic entity type that has a relationship with one or more subtypes Attribute Inheritance: Subtype entities inherit values of all attributes of the supertype An instance of a subtype is also an instance of the supertype

Subclasses and Superclasses (1) An entity type may have additional meaningful sub- groupings 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

Instances of a specialization.

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 real-world 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

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

Figure 4-1 Basic notation for supertype/subtype notation a) EER notation

Figure 4-1 Basic notation for supertype/subtype notation (cont.) b) Microsoft Visio Notation Different modeling tools may have different notation for the same modeling constructs

Figure 4-2 Employee supertype with three subtypes All employee subtypes will have emp nbr, name, address, and date-hired Each employee subtype will also have its own attributes

Relationships and Subtypes Relationships at the supertype level indicate that all subtypes will participate in the relationship The instances of a subtype may participate in a relationship unique to that subtype. In this situation, the relationship is shown at the subtype level

Figure 4-3 Supertype/subtype relationships in a hospital Both outpatients and resident patients are cared for by a responsible physician Only resident patients are assigned to a bed

Generalization and Specialization Generalization: The process of defining a more general entity type from a set of more specialized entity types. BOTTOM-UP Specialization: The process of defining one or more subtypes of the supertype and forming supertype/subtype relationships. TOP-DOWN

Figure 4-4 Example of generalization a) Three entity types: CAR, TRUCK, and MOTORCYCLE All these types of vehicles have common attributes

Figure 4-4 Example of generalization (cont.) b) Generalization to VEHICLE supertype So we put the shared attributes in a supertype Note: no subtype for motorcycle, since it has no unique attributes

Generalization. Two entity types, CAR and TRUCK. Generalizing CAR and TRUCK into the superclass VEHICLE.

Specialization 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

Example of a Specialization

Figure 4-5 Example of specialization a) Entity type PART Only applies to manufactured parts Applies only to purchased parts

Figure 4-5 Example of specialization (cont.) b) Specialization to MANUFACTURED PART and PURCHASED PART Note: multivalued attribute was replaced by an associative entity relationship to another entity Created 2 subtypes

Notation for Specialization Employee Secretary Technician Engineer d Work For Department Belongs To Professional Organization

Constraints on Specialization & Generalization Several specializations can be defined on an entity type. Entities may belong to subclasses in each of the specializations. The specialization may also consist of a single subclass, such as the manager specialization, in this case we don’t use the circle notation. Types of Specializations Predicate-defined or Condition-defined specialization Attribute-defined specialization User-defined specialization

Types of Specializations: Predicate-defined If we can determine exactly those entities that will become members of each subclass by a condition, the subclasses are called predicate-defined (or condition-defined) subclasses Example: Income > 0 The condition is called the defining predicate of the sub class. The condition is a constraint specifying exactly those entities of the Employee entity type whose attribute value for Job Type is Secretary belong to the subclass. Predicate defined subclasses are displayed by writing the predicate condition next to the line that connects the subclass to the specialization circle

Types of Specializations: Attribute-defined If all subclasses in a specialization have membership condition on same attribute of the superclass, specialization is called an attribute defined- specialization Attribute is called the defining attribute of the specialization Example: JobType is the defining attribute of the specialization {SECRETARY, TECHNICIAN, ENGINEER} of EMPLOYEE Attribute-defined specializations are displayed by placing the defining attribute name next to the arc from the circle to the super class.

EER diagram notation for an attribute-defined specialization on JobType.

Types of Specializations: User-defined If no condition determines membership, the subclass is called user-defined Membership in a subclass is determined by the database users by applying an operation to add an entity to the subclass Membership in the subclass is specified individually for each entity in the superclass by the user

Constraints on Specialization and Generalization Two other conditions apply to a specialization/generalization: Disjointness Constraint: Specifies that the subclasses of the specialization must be disjointed (an entity can be a member of at most one of the subclasses of the specialization) Specified by ‘d’ in EER diagram If not disjointed, overlap; that is the same entity may be a member of more than one subclass of the specialization Specified by ‘o’ in EER diagram

EER diagram notation for an overlapping (nondisjoint) specialization.

Constraints on Specialization and Generalization Completeness Constraint: Total specifies that every entity in the superclass must be a member of some subclass in the specialization/ generalization Shown in EER diagrams by a double line Partial allows an entity not to (may or may not) belong to any of the subclasses Shown in EER diagrams by a single line

Constraints on Specialization and Generalization Hence, we have four types of specialization / generalization constraints: Disjoint, total Disjoint, partial Overlapping, total Overlapping, partial Note: Generalization usually is total because the superclass is derived from the subclasses. Disjointness vs. Completeness Disjoint constraints and completeness constraints are independent. The following possible constraints on specializations are possible:

Disjointness vs. Completeness Disjoint, total Department d Academic Administrative Disjoint, partial Employee d Secretary Analyst Engineer

Disjointness vs. Completeness Overlapping, total Part o Manufactured Purchased Overlapping, partial Movie o Children Comedy Drama

Constraints in Supertype/ Completeness Constraint (ONCE AGAIN) Completeness Constraints: Whether an instance of a supertype must also be a member of at least one subtype Total Specialization Rule: Yes (double line) Partial Specialization Rule: No (single line)

Figure 4-6 Examples of completeness constraints a) Total specialization rule A patient must be either an outpatient or a resident patient

Figure 4-6 Examples of completeness constraints (cont.) b) Partial specialization rule A vehicle could be a car, a truck, or neither

Constraints in Supertype/ Disjointness constraint (ONCE AGAIN) Disjointness Constraints: Whether an instance of a supertype may simultaneously be a member of two (or more) subtypes Disjoint Rule: An instance of the supertype can be only ONE of the subtypes Overlap Rule: An instance of the supertype could be more than one of the subtypes

Figure 4-7 Examples of disjointness constraints a) Disjoint rule A patient can either be outpatient or resident, but not both

Figure 4-7 Examples of disjointness constraints (cont.) b) Overlap rule A part may be both purchased and manufactured

Insertion & Deletion rules Deleting an entity from a superclass implies that it is automatically deleted from all the subclasses to which it belongs. Inserting an entity in a superclass means that the entity is mandatorily inserted in all predicate-defined (attribute defined) subclasses for which the entity satisfies the defining predicate. Inserting an entity in a superclass of total specialization implies that the entity is mandatorily inserted in at least one of the subclasses of the specialization. Number of other rules can be inferred for insertions & deletion can be derived from various types of specializations.

Example of __________ Specialization Disjoint Partial

Specialization / Generalization Hierarchies, Lattices and Shared Subclasses (01) A subclass may itself have further subclasses specified on it Forms a hierarchy or a lattice Hierarchy has a constraint that every subclass has only one superclass (called single inheritance) Lattice, a subclass can be subclass of more than one superclass (called multiple inheritance) In a lattice or hierarchy, a subclass inherits attributes not only of its direct superclass, but also of all its predecessor superclasses A subclass with more than one superclass is called a shared subclass

Specialization / Generalization Hierarchies, Lattices and Shared Subclasses (02) Can have specialization hierarchies or lattices, or generalization hierarchies or lattices In specialization, start with an entity type and then define subclasses of the entity type by successive specialization (top down conceptual refinement process) In generalization, start with many entity types and generalize those that have common properties (bottom up conceptual synthesis process) In practice, the combination of two processes is employed

Specialization/Generalization Lattices and Hierarchies (Example) Employee Secretary Technician Engineer Manager Salaried_Emp Hourly Emp Engineering Manager d In a lattice, when a superclass inherits attributes from more than one superclass, and some attributes are inherited more than once via different paths (i.e. Engineer, Manager and Salaried Employee all inherit from Employee, that are then inherited by Engineering Manager. In this situation, the attributes are included only once in the subclass

Figure 4-8 Introducing a subtype discriminator (disjoint rule) A simple attribute with different possible values indicating the subtype attribute defined-specialization

Figure 4-9 Subtype discriminator (overlap rule) A composite attribute with sub-attributes indicating “yes” or “no” to determine whether it is of each subtype Predicate-defined

Figure 4-10 Example of supertype/subtype hierarchy

Specialization / Generalization Lattice Example (UNIVERSITY)

Categories (UNION TYPES) All of the superclass/subclass relationships we have seen thus far have a single superclass A shared subclass is subclass in more than one distinct superclass/subclass relationships, where each relationships has a single superclass (multiple inheritance) In some cases, need to model a single superclass/subclass relationship with more than one superclass Superclasses represent different entity types Such a subclass is called a category or UNION TYPE

Example of categories (UNION TYPES) Database for vehicle registration, vehicle owner can be a person, a bank (holding a lien on a vehicle) or a company. Category (subclass) OWNER is a subset of the union of the three superclasses COMPANY, BANK, and PERSON A category member must exist in at least one of its superclasses Note: The difference from shared subclass, which is subset of the intersection of its superclasses (shared subclass member must exist in all of its superclasses).

Formal Definitions of EER Model (1) Class C: A set of entities; could be entity type, subclass, superclass, category. Subclass S: A class whose entities must always be subset of the entities in another class, called the superclass C of the superclass/subclass (or IS-A) relationship C/S: S ⊆ C Specialization Z: Z = {S1, S2,…, Sn} a set of subclasses with same superclass G; hence, G/Si a superclass relationship for i = 1, …., n. G is called a generalization of the subclasses {S1, S2,…, Sn} Z is total if we always have: S1 ∪ S2 ∪ … ∪ Sn = G; Otherwise, Z is partial. Z is disjoint if we always have: Si ∩ S2 empty-set for i ≠ j; Otherwise, Z is overlapping.

Formal Definitions of EER Model (2) Subclass S of C is predicate defined if predicate p on attributes of C is used to specify membership in S; that is, S = C[p], where C[p] is the set of entities in C that satisfy p A subclass not defined by a predicate is called user-defined Attribute-defined specialization: if a predicate A = ci (where A is an attribute of G and ci is a constant value from the domain of A) is used to specify membership in each subclass Si in Z Note: If ci ≠ cj for i ≠ j, and A is single-valued, then the attribute-defined specialization will be disjoint. Category or UNION type T A class that is a subset of the union of n defining superclasses D1, D2,…Dn, where n>1: T ⊆ (D1 ∪ D2 ∪ … ∪ Dn) A predicate pi on the attributes of T. If a predicate pi on the attributes of Di can specify entities of Di that are members of T. If a predicate is specified on every Di then : T = (D1[p1] ∪ D2[p2] ∪…∪ Dn[pn] Note: The definition of relationship type should have 'entity type' replaced with 'class'.

A specialization lattice with multiple inheritance for a UNIVERSITY database.

An EER conceptual schema for a UNIVERSITY database.

Entity Clusters EER diagrams are difficult to read when there are too many entities and relationships Solution: Group entities and relationships into entity clusters Entity cluster: Set of one or more entity types and associated relationships grouped into a single abstract entity type

Figure 4-13a Possible entity clusters for Pine Valley Furniture in Microsoft Visio Related groups of entities could become clusters

Figure 4-13a Possible entity clusters for Pine Valley Furniture in Microsoft Visio Related groups of entities could become clusters

Figure 4-13b EER diagram of PVF entity clusters More readable, isn’t it?

Figure 4-14 Manufacturing entity cluster Detail for a single cluster