1/23/2015
1
Lecture 2
Conceptual Modelling
The Entity-Relationship Model
Ref: Ramakrishnan and Gehrke 2003
Review of last lecture
•History of Data Management
•File-Based Systems
•Databases and DBMS vs File Systems
•Why Study Databases
•Data Model
•Structure of a DBMS
•Components of DBMS environment
•Roles in the Database Environment
1/23/2015 2
University of Botswana. CSI262 Lecture
Notes 2014/2015
Database system development stages
Stage 1 Requirements
- Determine what database is required to do (both current and
future needs)
- Develop data model
Stage 2 Design
- Tables
- Relationships
- Indexes
- Other structures
Stage 3 Implement
- Create database
- Create tables and relationships
- Create other structures
- Input data
- Test database
1/23/2015 3
University of Botswana. CSI262 Lecture
Notes 2014/2015
Overview of Database Design
•Conceptual design: (ER Model is used at this
stage.)
–What are the entities and relationships in the
enterprise?
–What information about these entities and
relationships should we store in the database?
–What are the integrity constraints or business rules
that hold?
– A database `schema’ in the ER Model can be
represented pictorially (ER diagrams).
–Can map an ER diagram into a relational schema.
1/23/2015 University of Botswana. CSI262 Lecture
Notes 2014/2015 4
ER Model Basics
•Entity: Real-world object distinguishable from
other objects. An entity is described (in DB) using
a set of attributes.
•Entity Set: A collection of similar entities. e.g., all
employees.
– All entities in an entity set have the same set of
attributes. (Until we consider ISA hierarchies,
anyway!)
–Each entity set has a key.
–Each attribute has a domain.
1/23/2015 University of Botswana. CSI262 Lecture
Notes 2014/2015 5
Student
SId FName Surname
ER Model Basics
1/23/2015 University of Botswana. CSI262 Lecture
Notes 2014/2015 6
Student
SId FName Surname
Course
CId CName Grade
• Relationship: Association among two or more entities. e.g., John
is enrolled for CSI362.
• Relationship Set: Collection of similar relationships.
- An n-ary relationship set R relates n entity sets E1 ... En;
each relationship in R involves entities e1 Є E1, ..., en Є En
- Same entity set could participate in different relationship
sets, or in different “roles” in same set.
Employee
supervisor subordinate
Enrolled_In
Reports_to
EName EId
