The EntityRelationship Model Part I. Instructor: Mohamed Eltabakh [email protected] CS3431: C-Term 2013 1 Database Design Stages Application Requirements Conceptual Design Conceptual Schema Logical Design Logical Schema Physical Design Physical Schema 2 Conceptual Design
What is Conceptual Design? Concise representation of the DB application requirements Why Conceptual Design ? It helps to understand application requirements better It helps to communicate our understanding of application It helps to come up with a good design 3 Conceptual Design (Contd) Conceptual Models
ER Model ER (Entity-Relationship) Model, Our focus UML (Unified Modeling Language), ORM (Object Role Modeling), etc. Structures: Entities and Relationships Constraints An ER schema is represented using EntityRelationship Diagram (ERD) 4 Modeling A database can be modeled as:
Collection of entities, Relationship among entities. An entity is an object that exists and is distinguishable from other objects. Example: a person, company, event, bank account, store, etc. Entities have attributes Example: a person have names and addresses An entity set is a set of entities of the same type that share the same properties.
Example: set of all persons, companies, trees, holidays 5 ER: Entity Sets and Attributes Entity set : Represent a set of objects Attribute: property of an entity, has a domain In ER diagram Entity set rectangle Attribute Oval. Entity Set Student with attributes (sNumber, sName, sAge) sNumber sName Student sAge 6
Attributes Types Primitive attributes: Attribute stores a single value (Number, String, Boolean, Date, ) Composite attributes: An attribute can be divided into subattributes (each is primitive) Multi-values attributes: Attribute with many values Derived attributes: Attributes computed from others sNumber sName Student These are primitive attributes
sAge DoB 7 Complex Attributes Composite Attribute: address s tre e t c ity Multivalued Attribute: major s ta te major a d d re s s Student entity set with all its attributes Derived Attribute: Age sN um ber sN am e
m a jo r S tu d e n t DoB Age s Age Age a d d re s s s tre e t c it y s ta te 8 Relationship Types
Relationship: Association (connection) between entities sets Relationship Type: Class of relationships Representation: Use a diamond shape Relationship type HasTaken to represent Courses taken by Students sNumber cNumber Student sName Has Taken Course title 9 Relationships with Attributes
A relationship may have attributes These attributes do not belong to any of the connected entities. But they belong the relationship sNumber cNumber Student Has Taken Course sName title Relationship HasTaken has an attribute project which is the project the Student did for the Course sNumber cNumber Student
Has Taken sName Course title project 10 Example I (Simple Application) Suppliers have a name, and location. Products have a name, price, and number. Consumers have a name, and location. Suppliers supply products on certain dates, while consumers buy products of certain quantity How would you model this application? a) What are the entities and attributes? b) What are the relationships? 11
Modeling of Example I pNumbe r sName sPrice product date quantity supplies buys sName cName supplier consumer
sLoc cLoc This ER captures exactly what is written in text 12 Example II (More Complicated) Suppliers have a name, and location. Products have a name, and number. Consumers have a name, and location. Some Suppliers have established contracts to supply a certain Product to a particular Consumer for specially negotiated price at a given quantity. How would you model this application? 13 Modeling of Example II Model the relationship Supplier supplies Products to Consumers pNumber pName
15 ER Model so far Entities and entity sets Relationships Binary, ternary, multi-way Attributes For entity sets and relationship types Simple, composite, multi-valued, derived 17 What about an Exercise Assume
you are building a database for a Bank What are the entity sets, attributes, and relationships?? 18
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