Cơ sở dữ liệu - Chapter 3: The enhanced entity - Relationship (eer) model

Chapter 3 The Enhanced Entity - Relationship (EER) Model EER Model  The basic concepts of ER modeling are not powerful enough for some complex applications.  The Enhanced ER model is the extension of the original ER model with new modeling constructs. EER Model  Include all modeling concepts of basic ER  Additional concepts: ◦ Subclasses/superclasses ◦ Specialization/generalization, ◦ Categories, attribute inheritance  ƒIt is used to model applications more completely and accurately if needed  It includes some object-oriented concepts, such as inheritance Outline  Subclasses, Superclasses and Inheritance  Specialization and Generalization  Constrains and Characteristics  Union Subclasses, Superclasses  In many cases an entity type has numerous subgroupings of its entities that are meaningful and need to be represented explicitly because of their significance to the database application.  Ex: EMPLOYEE may be further grouped into: ◦ SECRETARY, ENGINEER, TECHNICIAN,  Based on the EMPLOYEE’s Job ◦ MANAGER  EMPLOYEEs who are managers ◦ SALARIED_EMPLOYEE, HOURLY_EMPLOYEE  Based on the EMPLOYEE’s method of pay Subclasses, Superclasses  We call each of these subgroupings a subclass of the EMPLOYEE entity type, and the EMPLOYEE entity type is called the superclass for each of these subclasses.  These are called superclass/subclass (as well as simply class/subclass) relationships: ◦ EMPLOYEE/SECRETARY ◦ EMPLOYEE/TECHNICIAN ◦ EMPLOYEE/MANAGER ◦  These are also called IS-A relationships ◦ SECRETARY IS-A EMPLOYEE, TECHNICIAN IS-A EMPLOYEE, . Subclasses, Superclasses  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 Subclasses, Superclasses Subclasses, Superclasses  A salaried employee who is also an engineer belongs to the two subclasses: ◦ ENGINEER, and ◦ SALARIED_EMPLOYEE  A salaried employee who is also an engineering manager belongs to the three subclasses: ◦ MANAGER, ◦ ENGINEER, and ◦ SALARIED_EMPLOYEE  It is not necessary that every entity in a superclass be a member of some subclass Subclasses, Superclasses  An important concept associated with subclasses is that of type inheritance  An entity that is member of a subclass inherits ◦ All attributes of the entity as a member of the superclass ◦ All relationships of the entity as a member of the superclass Subclasses, Superclasses Subclasses, Superclasses  Example: ◦ In the previous slide, SECRETARY (as well as TECHNICIAN and ENGINEER) inherit the attributes Name, SSN, , from EMPLOYEE ◦ Every SECRETARY entity will have values for the inherited attributes ◦ Every SECRETARY entity will also keep all relationships Specialization  Specialization is the process of defining a set of subclasses of an entity type  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. Specialization  It may have several specializations of the same superclass  Example: Another specialization of EMPLOYEE based on method of pay is {SALARIED_EMPLOYEE, HOURLY_EMPLOYEE}.  The subset symbol on each line connecting a subclass to ϵ indicates the direction of the superclass/subclass relationship Specialization Specialization ◦ Attributes of a subclass are called specific or local attributes.  For example, the attribute TypingSpeed of SECRETARY ◦ The subclass can also participate in specific relationship types.  For example, a relationship BELONGS_TO of HOURLY_EMPLOYEE Specialization  There are two major reasons for including class/subclass relationship and specialization in a data model: 1. Certain attributes may apply to some but not all entities of the superclass (secretary subclass has local attribute Typing speed where engineer has eng_type) 2. some relationship types may be participate in only by entities that are members of the subclass (Hourly_employees are related to Trade_nuion via belongs_to) Specialization  In summary, the specialization process allows us to do the following: ◦ Define a set of subclass of an entity type ◦ Establish additional specific attributes with each subclass ◦ Establish additional specific relationship types between each subclass and other entity types or other subclasses Generalization  Generalization is the reverse of the specialization process  Several classes with common features are generalized into a superclass; ◦ original classes become its subclasses Generalization  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 Generalization Constraints on Specialization and Generalization  Two basic constraints can apply to a specialization/generalization: ◦ Disjointness Constraint ◦ Completeness Constraint Constraints on Specialization and Generalization  Disjointness Constraint: ◦ An entity can be a member of at most one of the subclasses of the specialization ◦ Specified by d in EER diagram Displaying an attribute-defined specialization in EER diagrams Constraints on Specialization and Generalization  Overlap: ◦ When the subclasses are not disjoint. ◦ The same entity may be a member of more than one subclass of the specialization ◦ Specified by o in EER diagram Example of overlapping total Specialization Constraints on Specialization and Generalization  Completeness Constraint: ◦ Total: every entity in the superclass must be a member of some subclass  Shown in EER diagrams by a double line ◦ Partial: an entity not to belong to any of the subclasses  Shown in EER diagrams by a single line Constraints on Specialization and Generalization  Four types of specialization/generalization: ◦ Disjoint, total ◦ Disjoint, partial ◦ Overlapping, total ◦ Overlapping, partial Constraints on Specialization and Generalization  Some general rules: ◦ Deleting an entity from s superclass implies that it is automatically deleted from all the subclasses to which it belongs ◦ 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/Generalization Hierarchies, Lattices  A subclass may itself have further subclasses specified on it  Hierarchy has a constraint that every subclass has only one superclass (called single inheritance); this is basically a tree structure  In a lattice, a subclass can be subclass of more than one superclass (called multiple inheritance) Shared Subclass “Engineering_Manager” Specialization/Generalization Hierarchies, Lattices  Leaf node is a class that has no subclasses of its own  A subclass with more than one superclass is called a shared subclass (multiple inheritance)  Notice that the existence of at least one shared subclass leads to a lattice, otherwise, it’s a hierarchy Specialization / Generalization Lattice Example (UNIVERSITY) Union  All of the superclass/subclass relationships we have seen so far origin from a single superclass  Sometimes we may need more than one superclass  In this case, the subclass will represent a collection of objects that is a subset of the UNION of distinct entity types  We call such a subclass a UNION TYPE Union  Example: In a database for vehicle registration, a vehicle owner can be a PERSON, a BANK (holding a lien on a vehicle) or a COMPANY. ◦ A UNION type called OWNER is created to represent a subset of the union of the three superclasses COMPANY, BANK, and PERSON Two categories (UNION types): OWNER, REGISTERED_VEHICLE Union  We can compare a UNION (OWNER) with shared subclass (ENGINEERING_MANAGER)  The latter is a subclass of each of the three superclass ENGINEER, MANAGER and SALARIED_EMPLOYEE, so an entity that us a member of ENGINEERING_MANAGER must exist in all three  This means that an engineering manager must be an ENGINEER, a MANAGER, and a SALARIED_EMPLOYEE  On the other hand, an entity that is a member of OWNER must exist in only one of the superclass Shared Subclass “Engineering_Manager” UNION  Attribute inheritance works more selectively in the case of UNION.  For example, OWNER entity inherits attributes of a COMPANY, a PERSON OR a BANK  A shared subclass such as ENGINEERING_MANAGER inherits ALL the attributes of its superclasses