CS 4347.501 (Database Systems) - Chapter 4: The Enhanced Entity-Relationship (EER) Model

Final design

- the _____ ______ of hierarchies or lattices resulting from either process may be identical; the only difference relates to the manner or order in which the schema superclass and subclasses were created during the design process. In practice, it is likely that a combination of the processes is employed.

Total vs. partial completeness constraint

- the second constraint on specialization is called completeness (or totalness) constraint which may be total or partial - Hence we have the following four possible constraints on specialization: - d, t - d, p - o, t - o, p

2 reasons for using class/subclass relationships and specialization

- #1: certain attributes may apply to some but not to all entities of the superclass entity type. A subclass is defined in order to group entities to which these attributes apply. The members of the subclass may still share the majority of their attributes with the other members of the superclass. For example, in Figure 4.1 the SECRETARY subclass has the specific attribute typing_speed, where the ENGINEER subclass has the specific attribute Eng_type, but these subclasses share their inherited attributes from the EMPLOYEE entity type - #2: some relationship types may be participated in only by entities that are members of the subclass. For example, if only HOURLY_EMPLOYEES can belong to trade union, we can represent that fact by creating the subclass HOURLY_EMPLOYEE of EMPLOYEE and relating the subclass to an entity type TRADE_UNION vis BELONGS_TO relationship type, as illustrated in Figure 4.1

Insertion and 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 implies that the entity is mandatorily inserted in all predicate-defined (or attribute-defined) subclasses for which the entity satisfies the defining predicate - inserting an entity in a superclass of a total specialization implies that the entity is mandatorily inserted in at least one of the subclasses of the specialization

Specialization lattice

- Figure 4.7 shows another ______________ _______ for more than one level. These may be part of a conceptual schema for a UNIVERSITY database. Notice that this arrangement would have been a hierarchy except for the STUDENT_ASSISTANT subclass, which is a subclass in two distinct class/subclass relationship - Review Figure 4.7 along with the detailed explanation in the textbook

Subtype

- The first EER model concept we take up is that of a _______ or subclass of an entity type. In many cases an entity type has numerous subgroupings or subtypes of its entities that are meaningful and need to be represented explicitly because of their significance to the database application. - For example, the entities that members of EMPLOYEE entity type may be distinguised further into SECRETARY, ENGINEER, MANAGER, TECHNICIAN, SALARIED, HOURLY EMPLOYEE and so on. - See Figure 4.1, 4.2 - An entity cannot exist in the database merely being a member of a subclass; it must also be a member of the superclass

Generalization

- We can think of a reverse process of abstraction in which we suppress the differences among several entity types, identify their common features, and generalize them into a single superclass of which the original entity types are special subclasses. - For example, consider the entity types CAR and TRUCK shown in Figure 4.3(a)

Categories

- We display __________ in an EER diagram as shown in Figure 4.8. The superclasses COMPANY, BANK, and PERSON are connected to the circle with the U symbol, which stands for the set union operation. An arc with the subset symbol connects the circle to the subclass OWNER category. In Figure 4.8, we have two categories: OWNER which is a subclass (subset) of the union of PERSON, BANK, and COMPANY; and REGISTERED_VEHICLE which is a subclass of the union CAR and TRUCK - Can be total or partial.

Specialization hierarchy

- a ______________ _________ has the constraint that every subclass participates as a subclass in only one class/subclass relationship; that is each subclass has only one parent, which results in a tree structure or strict hierarchy. In contrast, for a specialization lattice, a subclass can be a subclass in more than one class/subclass relationship. Hence, Figure 4.6 is a lattice.

Disjoint vs. overlapping subclasses constraint

- attribute-defined specialization, and the defining attribute - SECRETARY, TECHNICIAN, ENGINEER in Figure 4.4 - implies the disjointness constraint like d in Figure 4.4 - the d is also applicable to user-defined like {HOURLY_EMPLOYEE, SALARIED_EMPLOYEE} - if the subclasses are not constrained to be disjoint, their sets of entities may be overlapping

Defining attribute

- if all subclasses in a specialization have their membership condition on the same attribute of the superclass, the specialization itself is called the ________ _________ of the specialization. When we don't have a condition for determining membership in a subclass, the subclass is user-defined

Total

- in general, a superclass that was defined through the generalization process usually is _____, because the superclass is derived form the subclasses and hence contains only the entities that are in the subclasses

Bottom-up conceptual synthesis (Bottom-up approach)

- it is possible to arrive at the same hierarchy or lattice from other direction. In such a case, the process involves generalization rather than specialization and corresponds to a ______-__ __________ _________. For example, the database designers may first discover entity type such as STAFF, FACULTY, ALMUNIS, ......

Union type (category)

- it is sometimes necessary to represent a collection of entities from different entity types. In this case, a subclass will represent a collection of entities that is a subset of the UNION of entities from distinct entity types, we call such a subclass a _____ ____. - this has 2 or more superclasses that may represent collections of entities from distinct entity types, where other superclass/subclass relationships always have 1 superclass. To better understand the difference, we can compare a category such as OWNER in Figure 4.8 with the ENGINEERING_MANAGER shared subclass in Figure 4.6. An entity that is a member of ENGINEERING_MANAGER must exist in all 3 collections {ENGINEERING, MANAGER, SALARIED_EMPLOYEE}; that is, the intersection of the 3 entity sets. On the other hand, an entity which is a member of the OWNER must exist in only one of the superclasses; either COMPANY, BANK, or a PERSON in Figure 4.8

Specialization

- it is the process of defining a set of subclass entity type; this entity type is called the superclass of ______________. The set of subclasses that forms ______________ is defined on the basis of distinguishing characteristics of the entities in the super class. - For example, the set of subclasses {SECRETARY, ENGINEER, TECHNICIAN} is a ______________ of the superclass EMPLOYEE that distinguishes among employee entities based on the job type of each employee. - Figure 4.1 shows how we represent a ______________ diagrammatically in an EER; Figure 4.2 shows a few entity instances that belong to subclasses of {SECRETARY, ENGINEER, TECHNICIAN} ______________ - A ______________ may also consist of a single subclass only, such as the {MANAGER} specialization in Figure 4.1; in such a case, we do not use the circle notation

Top-down conceptual refinement

- when designing the specialization in Figure 4.7, we may first specify an entity type PERSON for a UNIVERSITY database, then discover that 3 types of persons will be represented in the database: employees, alumni, and students; and we create the specialization {EMPLOYEE, ALMUNIS, STUDENT}. The overlapping constraint is chosen because a person may belong to more than one of the subclasses. We specialize EMPLOYEE into {STAFF, FACULTY, STUDENT_ASSISTANT}, and STUDENT into {GRADUATE_STUDENT, UNDERGRADUATE_STUDENT}. Finally, we specialize STUDENT_ASSISTANT into {RESEARCH_ASSISTANT, TEACHING_ASSISTANT}. This process is called ___-____ __________ __________. So far, we have a hierarchy; then we realize that STUDENT_ASSISTANT is a shared subclass, since it is also a subclass of STUDENT, leading to a lattice.

Chapter 4 introduction

designers of database applications have tried to design more accurate database schemas that reflect the data properties and constraints more precisely. This was particularly important for newer applications of database technology, such as databases for engineering design and manufacturing (CAD), telecommunications, complex software systems, GISs, etc. These types of databases have requirements that are more complex than the traditional applications. This led to the development of additional semantic data modeling concepts that were incorporated into conceptual data models such as ER model.