1. Conceptual Data Modeling, Entity Relationship Diagrams

2. Importance of Conceptual Data Modeling
Data rather than processes are more complex in many modern information systems.
Characteristics of data (structure, properties) are more stable, i.e. less likely to change over time, easier to reach consensus on.
It is shared between many processes, therefore is crucial in the design of databases, ensuring integrity of the data in an information system, efficiency of processing.

3. An Entity
An ENTITY or (Entity Type) is something of interest in the environment (e.g., person, place, object, event, concept), characterized with ATTRIBUTES.
Represented in E-R diagram by a rectangle
An ENTITY INSTANCE is a particular occurrence of an entity type – NOT SHOWN ON ERDs.
ENTITY ~ Table
ATTRIBUTE ~ Column in a table
ENTITY INSTANCE ~ Row in a table

4. An Attribute A discrete data element
A characteristic (property) of an entity
CUSTOMER
Customer_Number
Last_Name
First_Name
Street_Address
City
State
Zip
Phone
This Customer entity 
has eight attributes

5. Example: identify Entity, Attributes, Instances
Customer
Cust_ID Last_Name First_Name Address City ST Zip
Snerd Mortimer General Delivery Tampa FL
Fogg Bob Fogg Lane Omaha NE
Amos Famous Cookie Ct Miami FL
Targa Maxine Fast Lane Clinton NJ
George Scott Neat St Boulder CO
Guy Nice Pleasant St. Tampa FL
Smith Bob Quaker Path Wynn NY
Smith James Bayview Tampa FL

6. What Should an Entity Be?
SHOULD BE:
An object that is important to business
An object that will have many instances in the database
An object that will be composed of multiple attributes
SHOULD NOT BE:
Data that is not used by the application, should not be stored in the database.

7. Types of Attributes Stored vs Derived (e.g. DateOfBirth vs Age)
Simple vs. Composite
Simple - most basic level
Composite – decomposable into a group of related attributes
ex: address (street, city, state, zip)
Single Valued vs. Multi Valued –
Single - only one value per entity instance (e.g., last name, date of birth)
Mulitvalued- multiple values per entity instance (e.g., degrees, clubs, skills)
Stored vs Derived (e.g. DateOfBirth vs Age)

8. Attributes notation
Simple

9. An attribute broken into component parts
Textbook’s notation
An attribute broken into component parts
Multivalued
an employee can have
more than one skill
Derived
from date employed and current date 9

10. Alternative notation
Multivalued attributes are shown as double ellipses
Composite attributes may be shown broken down into their simple components
Simple/Single Valued; identifier
Multivalued
EMPLOYEE
emp-id
name
skill
f_name
l_name
m_name
composite

11. Example: time stamping37

12. Identifier Attributes
Every instance of an entity must be uniquely identified (to unambiguously distinguish them)
Simple identifier consists of one attribute
Composite identifier consists of more than one attribute (e.g., first name, middle name, and last name)
Partial identifier (in weak entities) – attribute that together with some attribute from another entity identifies an instance
Underline identifiers in diagrams
Double underline partial identifiers

13. Identifiers
Value of identifier must be unique for each entity instance
Should not change value over time
Guaranteed to have a valid value
No intelligent identifiers (e.g. containing locations or people that might change)
Consider substituting single-attribute artificial identifiers for natural composite identifiers to simplify design and enhance performance

14. Simple vs composite identifiyer

15. Relationships
A relationship is an association between one or more entities
The degree of a relationship indicates the number of entities involved
The cardinality of a relationship describes the number of instances of one entity associated with another entity

16. Cardinality Constraints
Cardinality Constraints - the number of instances of one entity that can or must be associated with each instance of another entity.
0 Optional relationships
0 or one
0 or many
Mandatory relationships
one and only one
one or more

17. Example
An employee can be assigned to any number of projects, or may not be assigned to any at all
A project must be assigned to at least one employee, and may be assigned to many

18. Degrees of Relationships: unary, binary, ternary
The number of different entities involved in a relationship

19. More on Relationships
Relationships (many-to-many or one-to-one) can have attributes
These describe features pertaining to the association between the entities in the relationship
Two entities can have more than one type of relationship between them (multiple relationships)
Associative Entity = combination of relationship and entity

20. Entities can be related to one another in more than one way
Figure 3-21a Employees and departments 40

21. Degrees of Relationships - ternary
A vendor supplies parts to warehouses. The unit cost and delivery method may differ for every warehouse.
Note: a relationship can have attributes of its own 24

22. More on Entities: Strong vs. Weak Entities
Strong entities
exist independently of other types of entities
has its own unique identifier
represented with single-line rectangle
Weak entity
dependent on a strong entity…cannot exist on its own
Does not have a unique identifier
represented with double-line rectangle
Identifying relationship
links strong entities to weak entities
represented with double line diamond

23. Associative Entities
Associative entities provide details of a many-to-many association.
It’s an entity – it has attributes
AND it’s a relationship – it links entities together
When should a many-to-many relationship with attributes instead be converted into an associative entity?
The associative entity could have meaning independent of the other entities
The associative entity preferably has a unique identifier, and should also have other attributes
The associative entity may participate in other relationships other than the entities of the associated relationship

24. Example:many-to-many relationship with attributes vs
Example:many-to-many relationship with attributes vs. associative entity 21

25. Associative entity – other notation
Associative entity is depicted as a rectangle with a diamond inside. 21

26. Modeling a ternary relationship as an associative entity36

27. SUMMARY of basic E-R notation8

28. Alternative ER Model Notation
Entity
Attribute
Relationship 2