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Understanding the Entity-Relationship Model in Database Design

This document explores the Entity-Relationship (ER) model, a vital concept in database design. It covers entities, attributes, and relationships, emphasizing their role in creating a comprehensive data model. The ER diagram serves as a blueprint for developing a logical and physical database schema, ensuring an organized representation of an organization’s data. The chapter details strong and weak entities, unique and composite attributes, and the importance of primary keys. Relationships among entities, including associative relationships, are also analyzed for effective data management.

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Understanding the Entity-Relationship Model in Database Design

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  1. The Entity-Relationship Model(Chapter 3 Additional Notes – Part a) Laku Chidambaram W.P. Wood Professor of MIS University of Oklahoma

  2. Overview • Database Design: The E-R Model • Entities • Attributes • Relationships

  3. General Design (Comprehensive Data Model) Conceptual Model (ERD) Blueprint (Database Schema) Logical Model (Relational) Aircraft (Database) Physical Reality (DBMS) Database Design Pilot (Order Entry) Engineer (A/C Receivable) Passenger (Inventory) External View

  4. The Entity Relationship Diagram • An E-R diagram is a graphical representation of an organization’s data • Such data includes the people, places, objects, events, or concepts (along with their characteristics and relationships) that are relevant to an organization

  5. Entities • Person, place, object, event, or concept about which the organization wishes to collect and maintain data • Nouns are used to describe entities • Examples: • Employee, Student, Nurse • City, State, Country • Sale, Registration, Account • Entity Type versus Entity Instance

  6. Types of Entities • Strong Entity: can exist independently • Weak Entity: cannot exist without the “owner” entity (with which it has an identifying relationship) • Has a primary key that is derived from the owner entity in the relationship EMPLOYEE DEPENDENT EMPLOYEE has

  7. Attributes • Properties or characteristics of entities (or relationships) • Provide the actual data that describe entities (or relationships) EMPLOYEE E_ID E_Name E_Address

  8. Unique Attributes • Candidate keys are ALL attributes that uniquely define an instance of an entity • A primary key is the candidate key that is chosen as the “most” unique of them all! • A primary key (or identifier) is ideal if it: • does not change over time • has unique values that are not null • does not use “intelligent” features • (Note: may need to substitute simple keys for long, i.e., composite, keys)

  9. Composite Attributes • Attributes that are not “atomic” (or simple), i.e., those attributes that can (and need to) be broken down further EMPLOYEE E_ID E_Name E_Address (E_Street, E_City, E-Zip)

  10. Multi-valued Attributes • Some attributes may have multiple values ... • … but they may need to be “fixed” before proceeding EMPLOYEE {E_Skill} EMPLOYEE SKILL S_Code S_Name has

  11. Derived Attributes • Those attributes whose values can be calculated from the values of other attributes (either in the database or in the system). • Value is not stored in the database (since it does not take up any space) • E.g., Age can be calculated, where date-of-birth is stored

  12. Relationships • Refer to the associations (or links) between entities • Use verbs to describe the links • May have attributes (in some cases) • Are directional STUDENT DORMITORY Lives in EMPLOYEE STORE STORE Works in

  13. Associative Entities • A (many-to-many) relationship that is better represented as an entity because it: • contains attributes from the entities it links and has its own attributes • has independent meaning to end-users CUSTOMER PRODUCT Orders CUSTOMER ORDER PRODUCT

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