Lecture Database management systems Chapter 2 Entity relationship model. This chapter includes contents Introduction, overview of database design, ER model basics, symbols used in ER diagram, attribute classification, ER diagram, relationship degree, relationship classification,...
Chapter Entity Relationship Model Introduction Modeling databases using a graphical technique is proposed so that humans can relate to easily ER Model is based on a perception of a real world that consists of collection of basic objects called entities and relationships among these objects Overview of Database Design Conceptual design: (ER Model is used at this stage.) A database “schema” in the ER Model can be represented pictorially (ER diagrams) The next step in DB design is the actual implementation of the database, using a commercial DBMS By using the DBMS, conceptual design from high-level data model will be transformed into the implementation data model COMPANY example The COMPANY database keeps track of company’s employee, departments, and projects Suppose that after the requirement collection and analysis phase, the database designers provide the following description: COMPANY example The company is organized into DEPARTMENTs Each department has a name, number and an employee who manages the department The start date of the department manager will be kept track A department may have several locations Each department controls a number of PROJECTs Each project has a unique name, unique number and is located at a single location COMPANY example We store each EMPLOYEE’s name social security number, address, salary, sex, and birthdate Each employee works for one department but may work on several projects We keep track of the number of hours per week that an employee currently works on each project We also keep track of the direct supervisor of each employee Each employee may have a number of DEPENDENTs For each dependent, we keep track of their name, sex, birthdate, and relationship to the employee ER Model Basics ER model describes data relationships, and attributes as entities, Entity: an object that exists and is distinguishable from other objects In other words, the entity can be uniquely identified ◦ Ex: A particular person, for example Dr Alain is an entity A particular department, for example CSE faculty A particular place, for example HCM city can be an entity ER Model Basics Entity type or entity set: collection of similar entities ◦ Ex: All students in CSE, say STUDENT All courses in CSE, say COURSE An entity may belong to more than one entity type ◦ Ex: The same person is a LECTURER at one instance and STUDENT at another instance ER Model Basics Relationship: associations among two or more entities Ex: ◦ An Dang works in Pharmacy department ◦ Teaches is the relationship type between LECTURER and STUDENT Symbols Used in ER Diagram Entity sets Attributes Relationships Relationship degree Binary Relationship: two entities are involved Staff Is assigned Department Relationship degree Quaternary Relationship: two entities are involved Location Staff Is assigned Department Relationship degree Quaternary Relationships: involve entities SLIDES PROFESSOR Teaches COURSE STUDENT Relationship Classification One-to-Many Relationship Type: ◦ The relationship that associates one entity to more than one entity is called one-to-many relationship ◦ Symbol: ◦ Example: parent–child relationship For one parent there can be more than one child DEPARTMENT EMPLOYEE Relationship Classification One-to-One Relationship Type: ◦ ◦ ◦ ◦ a special case of one-to-many relationship True one-to-one relationship is rare Symbol: Ex: The relationship between the President and the country is an example of one-to-one relationship PRESIDENT COUNTRY Relationship Classification Many-to-Many Relationship Type ◦ Ex: The relationship between students and courses ◦ Symbol: Relationship Classification Many-to-One Relationship Type ◦ Ex: The relationship between EMPLOYEE and DEPARTMENT is an example of manyto-one relationship There may be many EMPLOYEES working in one DEPARTMENT Relationship Classification Reducing ER Diagram to Tables Mapping Entities: ◦ Regular entities Entity name = Name of the relation or table Attributes of ER diagram=Column name of the table Reducing ER Diagram to Tables Mapping Composite Attribute: CUSTOMER (Customer-ID, Customer name, Street, City, State, Zip) Reducing ER Diagram to Tables Mapping Multivalued Attribute: EMPLOYEE (Employee-ID, Name, Address) EMPLOYEE-SKILL (Employee-ID, Skill) Reducing ER Diagram to Tables Mapping one-to-Many Relationship: EMPLOYEE (Customer-ID, Name, Address) ORDER (Order-ID, Order-Date, Customer-ID) Reducing ER Diagram to Tables Mapping one-to-Many Relationship: EMPLOYEE (Customer-ID, Name, Address) ORDER (Order-ID, Order-Date, Customer-ID) Reducing ER Diagram to Tables Mapping Many-to-Many Relationship: EMPLOYEE(Customer-ID, Name, Other attributes) SHIPMENT(Shipment-No, Customer-ID, Vendor-ID, Date Amount) ORDER (Vendor-ID, Address, Other Attributes) Relationship Classification Ex: ◦ Employee – Dependent people ◦ Customer – Orders ◦ Goods – Vendors ... underlined Relationship degree Unary Relationship (recursive relationship) : ◦ In the unary relationship the number of associated entity is one An entity related to itself is known as recursive relationship. .. department ◦ Teaches is the relationship type between LECTURER and STUDENT Symbols Used in ER Diagram Entity sets Attributes Relationships ER Model Basics Attributes: properties of entity Ex: Roll number,... city can be an entity ER Model Basics Entity type or entity set: collection of similar entities ◦ Ex: All students in CSE, say STUDENT All courses in CSE, say COURSE An entity may belong