1. Database Design Principles – Lecture 3
Conceptual Database Design – identifying entities

2. Lecture Objectives
Discovering Entities

3. Discovering Entities
Using a top down approach, identify the major ‘things’ a business needs to store information about == entities
Document these in a conceptual model
Model will show the following:
Entity – something about which someone wants to store data; typically a person, a place, a thing, a concept, or an event

4. Discovering Entities Requirements gathering – focusing on data:
Interview end users
Review existing documentation (forms, reports)
Brainstorming
Overview of the business
Questionnaires
Review system documentation
Functional/non-functional requirements
Work flow diagrams
Data flow diagrams
Use case descriptions

5. Discovering Entities Use a technique like ‘noun filtering’
Look for nouns that describe data
Similar nouns can be grouped into an entity
Some nouns will become attributes

6. Discovering Entities
Generally, nouns translate into entities

7. Sample Conceptual Model - ERD
This is a preliminary conceptual model.
Model shows entities without attributes or relationships.

8. Entity
Corresponds to a table and not to a row in the relational environment
Can be strong or weak
Entity name is a noun

9. Entity Entity can be strong or weak
Strong Entity (existence independent)
Does not depend on the
existence of some other
entity to exist
Each entity occurrence
of a strong entity is uniquely
identifiable using the
primary key attributes of
that entity type
Weak relationship
(non-identifying)
Can identify each Plan based on a
plan code and can identify each
Client by a client code. Therefore, both are strong entities.

10. Entity Entity can be strong or weak Weak Entity (existence dependent)
Does depend on the
existence of some other
entity in order to exist
Cannot identify each
occurrence of the
Dependent. We can only
Identify the Dependent by
Knowing the Parent,
through the primary
key of the Parent.
Can identify each parent based on a primary key (therefore it is a strong entity type) and can only identify each dependent only by knowing the Parent. Therefore dependent is a weak entity.

11. Discovering Relationships
Verbs translate into relationships among entities
Relationships are bi-directional
Need to know the following:
Optionality
Cardinality

12. Relationship Participation
Optional participation
One entity occurrence does not require corresponding entity occurrence in particular relationship
Mandatory participation
One entity occurrence requires corresponding entity occurrence in particular relationship

13. Relationship Participation

14. Relationship Participation

15. The Entity Relationship Model
Widely accepted and adapted graphical tool for data modeling
Introduced by Chen in 1976
Graphical representation of entities and their relationships in a database structure

16. The Entity Relationship Model

17. The Entity Relationship Model

18. In Summary Steps to create a conceptual model Identify entities
Identify relationships between entities taking into consideration the business rules

19. Discovering Business Rules
Constrain some part of the business
For example:
An employee can only choose one benefit plan
An employee may or may not have dependents
An employee may opt out of a benefit

20. Discovering Business Rules
Sources of Business Rules:
Company managers
Policy makers
Department managers
Written documentation
Procedures
Standards
Operations manuals
Direct interviews with end users

21. Summary Entity relationship (ER) model
Uses ERD to represent conceptual database as viewed by end user
ERD’s main components:
Entities
Relationships
Includes connectivity and cardinality notations
Connectivities and cardinalities are based on business rules
In ERD, M:N relationship is valid at conceptual level

22. Create an ERD for the following:
A department employs many employees, but each employee is employed by one department.
A division operates many departments, but each department is operated by one division.
An employee may be assigned to many projects, and a project may have many employees assigned to it.

23. Create an ERD for the following:
Identify the entities
Draw the ERD showing relationships
A Librarian wishes to make inquiries about borrowing activity within a library. The librarian can inquire about specific books borrowed by a customer using either their account or name. An inquiry can also be made by book category (such as fantasy, children’s, reference, etc) or an inquiry can be made that will look at the titles of a specific book.

24. Create an ERD for the following:
Identify the entities
Draw the ERD showing relationships
Multiple Real Estate Listing Service (MRELS) is a company that has offices throughout Southern Ontario. These offices have a number of Real Estate agents working for them. MRELS deals in both residential and commercial real estate. Agents work with customers to get a listing or to find a listing for a customer. A listing will include the address and the type of property (residential or commercial), square footage, annual taxes and closing date. If it is a residential property, it will include number of bedrooms, number of bathrooms, special features (central air, heated by gas or oil, radiators or forced air, for instance). For a commercial property, it will include number of offices as well as number of docking bays (where trucks back up to be loaded/unloaded).