CSE 480 Database Systems Lecture 4 Enhanced EntityRelationship

CSE 480: Database Systems Lecture 4: Enhanced Entity-Relationship Modeling Reference: Read Chapter 8. 1 – 8. 5 of the textbook 1

Announcements l Class Project – Each group must email the instructor by next Tuesday your group members u If your group has only 2 members, state whether you’re willing to accept a third member – If you don’t have a group yet, email me too. I’ll try to set up one for you. 2

Why EER? l ER diagram has limitations in terms of accurately reflecting the data properties and constraints l EER, which stands for Enhanced ER includes all the modeling concepts of basic ER with the following additional concepts – – subclasses/superclasses type inheritance shared subclasses and multiple inheritance categories (UNION types) 3

Why Subclasses and Superclasses? l An entity type may have additional subgroupings: – EMPLOYEE may be further grouped into: u SECRETARY, ENGINEER, TECHNICIAN – Based on job type u SALARIED_EMPLOYEE, HOURLY_EMPLOYEE – Based on method of pay l EER diagram extends ER diagram to represent these additional subgroupings – Each subgrouping is called a subclass of EMPLOYEE – EMPLOYEE is the superclass of these subclasses 4

Is-A Relationships in EER l Subclass-superclass relationships are also called IS-A relationships – SECRETARY IS-A EMPLOYEE, TECHNICIAN IS-A EMPLOYEE, …. Subclass Superclass Subclass 1 Subclass 2 Superclass Subclass 3 5

Specialization l A superclass can have several specializations – EMPLOYEE has 3 specializations l Some specializations have only 1 subclass (e. g. , Manager) 6

Is-A vs 1 -to-1 Relationship MANAGES EMPLOYEE DEPARTMENT 1 EMPLOYEE 1 TECHNICIAN u In a 1 -1 relationship, two distinct entities are related u In a subclass-superclass relationship, the subclass member is the same entity as the superclass member but in a distinct specific role 7

Subclasses and Superclasses l A subclass entity must be a member of the superclass l But a superclass entity does not have to be a member of any of its subclasses 8

Why do we need Subclass/Superclass? l Certain attributes may apply to some but not all entities of the superclass – Ex: Typing speed is an attribute of SECRETARY but not a TECHNICIAN l Certain relationship types are applicable to some entities but not others – HOURLY_EMPLOYEE has a relationship with TRADE_UNION; SALARIED_EMPLOYEE does not have such a relationship 9

Attributes of a Subclass l Local attributes: Typing. Speed of SECRETARY, Tgrade of TECHNICIAN l Type inheritance: a subclass also inherits all attributes of its superclass – SECRETARY inherits the attributes Name, SSN, Birth_date and Address from the EMPLOYEE entity type (its superclass) 10

Relationships of a Subclass l A subclass can participate in specific relationship types – Ex: MANAGER subclass participates in the MANAGES relationship with PROJECT entity type 11

Relationships of a Subclass l A subclass also inherits all the relationships in which the superclass participates – If EMPLOYEE participates in WORKS_FOR relationship with DEPARTMENT then u u u SECRETARY also participates in WORKS_FOR relationship with DEPARTMENT ENGINEER also participates in WORKS_FOR relationship with DEPARTMENT TECHNICIAN also participates in WORKS_FOR relationship with DEPARTMENT MANAGER also participates in WORKS_FOR relationship with DEPARTMENT SALARIED_EMPLOYEE also participates in WORKS_FOR relationship with DEPARTMENT HOURLY_EMPLOYEE also participates in WORKS_FOR relationship with DEPARTMENT 12

Constraints on Is-A Relationships Subclass 1 Subclass 2 Superclass Subclass 3 l Constraint is defined on the subclass membership of the superclass entities – Disjointness constraint u How many subclasses a superclass entity can belong? (max) – Completeness constraint u Must a superclass entity belong to any of the subclasses? (min) 13

Disjointness Constraint l Specifies whether subclasses are disjoint: – an entity can be a member of at most one of the subclasses of the specialization superclass – Specified by “d” in EER diagram – Ex: STUDENT is either FRESHMAN, d SOPHOMORE, JUNIOR, SENIOR, GRADUATE l If not disjoint, specialization is overlapping: – an entity may be a member of more than one subclasses – Specified by “o” in EER diagram – Ex: PERSON can be STUDENT, EMPLOYEE, or both superclass o 14

Completeness Constraint l Total specialization – Every entity in superclass must be a member of at least one subclass in the specialization – Ex: every EMPLOYEE must be either an HOURLY_EMPLOYEE or SALARIED_EMPLOYEE superclass – Shown in EER diagrams by a double line l Partial specialization – Allows a superclass entity not to belong to any subclasses – Ex: some EMPLOYEE entities do not belong to any of the subclasses SECRETARY, TECHNICIAN, or ENGINEER – Shown in EER diagrams by a single line superclass 15

Example: Disjoint Partial Specialization Partial Disjoint 16

Example: Overlapping Total Specialization Total Overlapping 17

Exercise l l Can a technician be an engineer? Can a manager belong to a trade union? 18

Exercise: MOVIE database l l Each movie is identified by title and year of release. Each movie has a length in minutes and is classified under one or more genres (horror, action, drama, etc) Each movie has one or more directors and one or more actors appear in it. Each movie also has a plot outline and zero or more quotable quotes, each of which is spoken by a particular actor appearing in the movie Actors are identified by name and date of birth and appear in one or more movies. Each actor has a role in the movie Directors are also identified by name and date of birth and direct one or more movies. It is possible for a director to act in a movie (including one he or she may also direct) 19

Exercise: Movie Database role M ACTOR Name Do. B genres ID quotes ACTS N Title Person o ID Year MOVIE length N DIRECTOR M Plot_ Outline DIRECTS 20

Hierarchies & Lattices l Hierarchy (Single Inheritance) – every subclass has only one immediate superclass l Lattice (Multiple inheritance) – a subclass can be subclass of more than one immediate superclass – A subclass with multiple immediate superclasses is called a shared subclass l In a lattice or hierarchy, a subclass inherits attributes not only of its immediate superclass, but also of all its predecessor superclasses 21

Lattice (Multiple Inheritance) Shared Subclass 22

Another Shared Subclass Example l ENGINEERING_MANAGER is a shared subclass – ENGINEERING_MANAGER Is-a ENGINEER – ENGINEERING_MANAGER Is-a MANAGER – ENGINEERING_MANAGER Is-a SALARIED_EMPLOYEE l It will inherit all the attributes of its superclasses, including EMPLOYEE, ENGINEER, MANAGER, and SALARIED_EMPLOYEE 23

Categories (UNION types) l A shared subclass is a subclass participating in multiple superclass/subclass (is-a) relationships – Each distinct relationship has a single superclass l In some cases, we need to model a single superclass/subclass relationship with more than one superclass – Each superclass can represent a different entity type – Such a subclass is called a category or UNION TYPE l Difference between shared subclass and union type – An entity of a shared subclass exists in all of its superclasses – An entity of a category exists in only one of its superclasses u It inherits only attributes of the superclass in which it belongs 24

Categories (UNION types) OWNER is the subclass of the union of 3 superclass entity types, PERSON, BANK, COMPANY (which have different key attributes) 25

Example of UNION types OWNER is the subclass of the union of 3 entity types, PERSON, BANK, COMPANY (which have different key attributes) REGISTERED_VEHICLE is the subclass of the union of CAR and TRUCK (which have same key attribute) 26

Constraints on UNION types l Total vs partial category (see example on next slide) – Total category holds the union of all entities in its superclasses u Represented by a double line – Partial category holds a subset of the union u Represented by a single line 27

Total vs Partial Category Unregistered Registered u User is the union of all registered and unregistered user entity types Owner is the union of all banks, but not all persons and not all companies 28

Summary l Introduced the EER model concepts – Class/subclass relationships – Type inheritance – Shared subclass and categories (union types) l These augment the basic ER model concepts introduced in lectures 2 -3 29