1. Primary Keys
Uniqueness of Table Rows
Identifier Attributes and Primary Keys
Composite Identifiers
Natural and Artificial Identifiers and Autogenerated Primary Keys
Candidate Keys and Alternate Keys
Problem Keys
Diagraming Tables with Key Attributes

2. Uniqueness of Table Rows
Remember our twin goals of minimizing storage space and reducing data redundancy
A duplicate record (occurrence) in a table violates this rule
Because any record represents one real world occurrence of the thing the entity defines if there is a duplicate record there must be a duplicate occurrence
But each occurrence is unique so duplicate records are wasted space—one record is enough

3. Uniqueness of Table Rows and Update Anomalies
An “update anomaly” occurs when data is not updated correctly
Duplicate records affect data accuracy through update anomalies and processing efficiency
Occur when duplicate data exists and not all matching records are updated when a change is needed
May be two records for the same customer with different addresses
All records must be checked (since there could be duplicates) instead of stopping when the matching record is found

4. A Theory for the Importance of Primary Keys
If a subset of attributes (including a single attribute) can be guaranteed to have unique values in a single table…
…then each record in the table is guaranteed to be unique…
…even if there are duplicates for the values of other attributes
Student ID is unique to each student
Roommates will share address attribute values

5. Identifier Attributes
Identifier Attribute(s): An attribute or set of attribute(s) whose value(s) uniquely identify each occurrence of an entity
Identifier Attributes are forced to be unique within a table
SSN for student or faculty
VIN for an automobile
SKU for a retail product
We will refer to Identifier Attributes at design time and these will become Primary Keys upon implementation (after some analysis)

6. Identifier Attributes Define Entities
A Vehicle Identification Number (VIN) identifies one and only one instance of an automobile
Since only an automobile can be identified by a VIN, an entity with VIN as an identifier attribute can only contain occurrences of automobiles
The VIN then identifies a car but defines the entity
What entities do these attributes define?
Social Security Number
Radio Call Letters (WMFE)
Universal Product Code (UPC)
Identifier attributes can never, ever be null

7. Composite Identifiers
Some entities need more than one attribute to form a unique value
DepartmentCode & CourseNumber
Both attributes combine to be unique and identify each occurrence and define the entity
Duplicates are allowed within each attribute
Any MIS course has an “ISM” prefix
There can be an English course with “4212” as the course number
The combination of values (“ISM 4212”) must be unique

8. Composite Identifiers (cont)
What is the natural PK of our section of this course?
Of the fact that you are enrolled in this course?
What entities are identified by the following values?
RegisterNumber & StartTime (DateTime value)
VIN and SSN
Building & RoomNumber (UCF database)
AptNum & StartDate
ProductID & SaleID

9. “Natural” and “Artificial” Identifiers
Most PKs are “artificial” or “generated”
SSN – StudentID
VIN – CourseNum
SerialNumber – LicensePlateNum & State
Some PKs are (more) “natural”
MagazineTitle & PublicationDate
Section Identifier PK
The distinction isn’t particularly important

10. Autogenerated PKs
An important class of artificial PKs are the anuogenerated, autonumber, or “Identity” attributes
DBMS can be configured to automatically generate a unique sequential number for each new record added to a table
By definition these values are unique and so can be a PK

11. Candidate Keys
Some entities may have more than one attribute or sets of attributes that can serve as a PK
VIN
ParkingDecalNum & Year
State & TagNum
How is a house identified?
Often a “natural” PK is cumbersome and an artificial PK, especially an autonumber, is introduced
One candidate key is selected to be the PK
Nonselected candidate keys become “alternate keys”

12. Some Problem PKs
People’s names are horrible choices for PKs
(Is there a “natural” PK for a person?)
Avoid PKs that change frequently
All things being equal, choose the simplest candidate key to be the PK
Always ensure that you PK defines the entity that you have in mind

13. A Data Model Segment

14. Documenting Identifier Attributes (cont.)
Identifier attributes are underlined in an ER diagram

15. PKs and Other Attributes in SQL Server Diagram
Composite PK
Single-attribute PK