Database Systems - Chapter 3: Enhanced Entity-Relationship Model - Trương Quỳnh Chi

Nội dung text: Database Systems - Chapter 3: Enhanced Entity-Relationship Model - Trương Quỳnh Chi

1. Contents 1 Introduction to Enhanced-ER Model 2 Subclasses, Superclasses, and Inheritance 3 Specialization and Generalization Constraints and Characteristics of Specialization and 4 Generalization Hierarchies 5 Categories 6 Design Choices, and Formal Definitions 2
2. Introduction to Enhanced-ER Model  Enhanced ER or Extended ER (EER) model  Created to design more accurate database schemas  More complex requirements than traditional applications  EER model includes all modeling concepts of the ER model. In addition, EER includes:  Subclasses and superclasses  Specialization and generalization  Category or union type  Attribute and relationship inheritance  EER diagrams  Diagrammatic technique for displaying these concepts in an EER schema 4
3. Subclasses, Superclasses & Inheritance  Subtype or subclass of an entity type  Subgroupings of entities that are meaningful  Represented explicitly because of their significance to the database application  Terms for relationship between a superclass and any one of its subclasses  Superclass/subclass  Supertype/subtype  Class/subclass  A class/subclass relationship is often called an IS-A (IS-AN) relationship. 6
4. EER diagram notation to represent subclasses & specialization 8
5. Subclasses, Superclasses & Inheritance  Subclass entity inherits all attributes and relationships of superclass  Notice that a subclass, with its own specific (or local) attributes and relationships together with all the attributes and relationships it inherits from the superclass, can be considered an entity type in its own right. 10
6. Specialization and Generalization  Specialization  Process of defining a set of subclasses of an entity type, called superclass  Defined on the basis of some distinguishing characteristic of the entities in the superclass  May have several specializations of the same entity type based on different distinguishing characteristics  Subclass can have its own:  Specific attributes (local attributes)  Specific relationship types 12
7. Instances of a specialization 14
8. Example of a Generalization 16
9. Specialization and Generalization  Data Modeling with Specialization and Generalization  A superclass or subclass represents a set of entities  Shown in rectangles in EER diagrams (as are entity types)  Sometimes, all entity sets are simply called classes, whether they are entity types, superclasses, or subclasses 18
10. Constraints and Characteristics of Specialization and Generalization Hierarchies  Constraints that apply to a single specialization or a single generalization  Differences between specialization/ generalization lattices and hierarchies 20
11. Constraints on Specialization & Generalization  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  Condition is a constraint that determines subclass members  Display a predicate-defined subclass by writing the predicate condition next to the line attaching the subclass to its superclass 22
12. EER diagram notation for an attribute-defined specialization on JobType 24
13. Constraints on Specialization & Generalization  Two basic conditions apply to a specialization/ generalization: disjointness and completeness constraints  Disjointness constraint  May be disjointed or overlap  Completeness (or totalness) constraint  May be total or partial  Disjointness and completeness constraints are independent 26
14. Constraints on Specialization & 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 belong to any of the subclasses: Shown in EER diagrams by a single line 28
15. Example of Overlapping Total Specialization 30
16. Case study: Identify superclass-subclass GROUP A GROUP B A system for course A system for a Library of a registration of Bach Khoa University University Hints: Students (Grad- Hints: Materials (Book, Student, undergrad- Journal, ), Borrowers student), Modules (for (Guest, Member) grad-student, for undergrad-students) 32
17. Specialization and Generalization Hierarchies and Lattices  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  In case of multiple inheritance, if attribute (or relationship) originating in the same superclass inherited more than once via different paths in lattice  Included only once in shared subclass 34
18. Utilizing Specialization and Generalization in Refining Conceptual Schemas  Specialization process  Start with entity type then define subclasses by successive specialization  Top-down conceptual refinement process  Bottom-up conceptual synthesis  Involves generalization rather than specialization 36
19. Categories  Category or Union type  Represents a single superclass/subclass relationship with more than one superclass  Subclass represents a collection of objects that is a subset of the UNION of distinct entity types  Attribute inheritance works more selectively  Category can be total or partial  Some modeling methodologies do not have union types 38
20. Two categories (union types): OWNER and REGISTERED_VEHICLE 40
21. Category vs. Shared Subclass 42
22. Category Shared Subclass A member entity of a category An member entity of a shared must exist in only one of its subclass must exist in all its superclasses (OR). superclasses (AND). • E.g.: An OWNER may be a COMPANY, a • E.g.: An engineering manager must be BANK, or a PERSON. an ENGINEER, a MANAGER, and a SALARIED_EMPLOYEE. A category is a subset of the A shared subclass is a subset of the union of its superclasses. intersection of its superclasses. • E.g.: OWNER is a subset of the union of • E.g.: ENGINEERING_MANAGER is a subset of the three superclasses. the intersection of the three superclasses. 44 Jan - 2015
23. Specialization/Generalization vs. Union Type 46
24. Contents 1 Introduction to Enhanced-ER Model 2 Subclasses, Superclasses, and Inheritance 3 Specialization and Generalization Constraints and Characteristics of Specialization and 4 Generalization Hierarchies 5 Categories 6 Design Choices, and Formal Definitions 48
25. Design Choices for Specialization/Generalization  If all the subclasses of a specialization/ generalization have few specific attributes and no specific relationships  Can be merged into the superclass  Replace with one or more type attributes that specify the subclass or subclasses that each entity belongs to 50
26. Formal Definitions for the EER Model Concepts  Class C  Set or collection of entities  Includes any of the EER schema constructs of group entities  Can be entity type, subclass, superclass, or category  Note: The definition of relationship type in ER/EER should have 'entity type' replaced with 'class‘ to allow relationships among classes in general 52
27. Formal Definitions for the EER Model Concepts  Specialization Z: Z = {S1, S2, , Sn} is a set of subclasses with same superclass G;  G/Si is a superclass/subclass relationship, i=1 n  G is called a generalization of the subclasses {S1, S2, , Sn}  Z is total if:  S1 ∪ S2 ∪ ∪ Sn = G;  Otherwise, Z is partial  Z is disjoint if:  Si ∩ Sj empty-set for i ≠ j;  Otherwise, Z is overlapping 54
28. Formal Definitions for the EER Model Concepts  Category or UNION type T  Class that is a subset of the union of n defining superclasses D1, D2, Dn, n>1: T ⊆ (D1 ∪ D2 ∪ ∪ Dn)  Can have a predicate pi on the attributes of Di to specify entities of Di that are members of T.  If a predicate is specified on every Di: T = (D1[p1] ∪ D2[p2] ∪ ∪ Dn[pn])  Relationship type  Any class can participate in a relationship 56
29. Exercise 2 WV Bank has three different types of bank accounts available for their customers. Each customer may have any number of bank accounts, and all accounts have a unique account number. For Savings Accounts, WV Bank must keep track of the account’s balance, interest rate, and the date the account was opened. Checking Accounts pay no interest, so the bank keeps track of just the balance and date opened. The third type of account, Loans, requires tracking the date the loan was taken, the balance due, and the interest rate of the loan. Loan Accounts are also assigned to a Loan Officer, who keeps track of the account and ensures that the client is making their payments on-time. 60