1. Database Design 2: Creating an ER Model
CS 320

2. Review: Steps in Creating an Entity-Relationship Model
Identify entities
Identify entity attributes and primary keys
Specify relationships

3. ER Model Notation Represent entities as rectangles
List attributes within the rectangle
Entity
Primary key:
Field whose value is
unique for each record
& serves as an identifier
Attributes

4. Aside: Tools for Creating ER Models
Microsoft Visio
Other open source tools
Drawing tools
Powerpoint
Illustrator

5. Data Model Naming Conventions
Entity names should be short, descriptive, compound words
UWEC_STUDENT, CandyProduct
Entity names will ultimately correspond to table names
Why compound words?
Avoids "reserved" words
Ensures every entity has a unique name

6. Data Model Naming Conventions (continued)
Attribute names should be descriptive compound words that correspond to the entity name
Attribute names will ultimately correspond to field names
Relates the field back to the table
Every attribute name within the database should be unique

7. Example Entity (Table) and Attribute (Field) Names
CANDY_CUSTOMER
CANDY_PURCHASE
CANDY_CUST_TYPE
CANDY_PRODUCT

8. Primary Key Attributes
Attribute whose value is unique for every entity instance
Every entity MUST have a PK
Designate by:
Placing as first attribute in the entity
Underline
Label using "PK"

9. Selecting Primary Keys
Must be values that are:
Unique for every possible record
Do not change
Best practice: numeric with no blank spaces or formatting characters
Often you need to create a surrogate key
ID value that serves only to identify the object in the database
Exception: objects with "natural" primary keys
SKU
ISBN
VIN

10. Atomic and Composite Attributes
Atomic attribute: represents a single data value
15, "Joline", 12/25/2009
Composite attribute: can be decomposed into atomic attributes
"James B. Brown"
"5580 Pinewood Road, Eau Claire, WI 54701"

11. Composite Attributes Always decompose into atomic components for:
Sorting
Searching
Formatting
Student_First_Name
Student_MI
Student_Last_Name
Student_Address_Line_1
Student_Address_Line_2
Student_City
Student_State
Student_Country
Student_Postal_Code

12. Multi-Valued Attributes
Attribute that might have multiple values for the same entity:

13. Modeling Multi-Valued Attributes
If it has a definite maximum number, leave as a repeating attribute
If the upper limit is variable, make a new entity and create a relationship

14. Derived Attributes Values that can be derived from other attributes
Student_Age = 22 (DOB = 11/20/1989, current date is 11/13/2011)
Order_Total = $500 (Item 1 cost = $200, Item 2 cost = $300)

15. Modeling Derived Attributes
In general, don't store derived attributes
Instead, create attributes to represent the underlying data values from which you can derive the value
Examples:
DOB => Age
Current_Price and Units_Sold of an item (for a sales order)

16. Data Model Relationships
Specify the number of instances of one entity that can be associated with instances of a related entity
Types:
1:M
1:1
M:M
“M” denotes some value greater than 1 whose upper bound is undetermined
This is called relationship cardinality

17. Example 1:M Relationship
Store_ID
Store_Name 1
Northside 2
Southside
Video_ID
Video_Title
Video_Format
1000
The Princess Bride
DVD
1001
Sideways
Bluray
1002
Just Visiting
1003
Crash

18. Example 1:1 Relationship
Spouse_ID
Spouse_Name 52
Ryan, Judy 53
Redmann, Rudy
Customer_ ID
Name
Customer_Address 1
Ryan, Paul
5454 Hyde Court 2
Myers, Mary
112 Birch Place

19. Example M:M Relationship

20. Putting it all together:

21. Summary: The Data Modeling Process
Define entities
Define attributes & primary keys
Define relationships

22. Test Yourself: Which of the following is the best choice for the primary key of a STUDENT entity?
STUDENT_NAME
STUDENT_
STUDENT_PHONE_NUMBER
STUDENT_SOC_SECURITY_NUMBER
None of the above are good choices

23. Test Yourself: Which of the following is the best choice for the primary key of a STUDENT entity?
STUDENT_NAME
STUDENT_
STUDENT_PHONE_NUMBER
STUDENT_SOC_SECURITY_NUMBER
None of the above are good choices

24. Test Yourself: In the video rental store database, VIDEO_ID is an example of a:
Primary key
Surrogate key
Multi-valued attribute
Derived attribute
Both a and b

25. Test Yourself: In the video rental store database, VIDEO_ID is an example of a(n):
Primary key
Surrogate key
Multi-valued attribute
Derived attribute
Both a and b

26. Your Turn: Draw the ER Model for the following database application:
Al’s Body Shop keeps data on customers that includes name,
address, phone number, car make, car model, car year, and VIN.
When a customer brings a car in for service, the service manager
completes a work order that contains the date of the service,
the name of each service performed, a description of each service,
and the charge for each service.

27. Solution