IMS 6217: Primary Key Reference 1 Dr. Lawrence West, MIS Dept., University of Central Florida 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

IMS 6217: Primary Key Reference 2 Dr. Lawrence West, MIS Dept., University of Central Florida 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

IMS 6217: Primary Key Reference 3 Dr. Lawrence West, MIS Dept., University of Central Florida 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

IMS 6217: Primary Key Reference 4 Dr. Lawrence West, MIS Dept., University of Central Florida 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

IMS 6217: Primary Key Reference 5 Dr. Lawrence West, MIS Dept., University of Central Florida 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)

IMS 6217: Primary Key Reference 6 Dr. Lawrence West, MIS Dept., University of Central Florida 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 also 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

IMS 6217: Primary Key Reference 7 Dr. Lawrence West, MIS Dept., University of Central Florida 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

IMS 6217: Primary Key Reference 8 Dr. Lawrence West, MIS Dept., University of Central Florida 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

IMS 6217: Primary Key Reference 9 Dr. Lawrence West, MIS Dept., University of Central Florida “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

IMS 6217: Primary Key Reference 10 Dr. Lawrence West, MIS Dept., University of Central Florida 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 –Identity attribute value unique within any table –Global Unique Identifier (GUID) unique anywhere in the world By definition these values are unique and so can be a PK

IMS 6217: Primary Key Reference 11 Dr. Lawrence West, MIS Dept., University of Central Florida 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”

IMS 6217: Primary Key Reference 12 Dr. Lawrence West, MIS Dept., University of Central Florida Some Problem PKs People’s names are horrible choices for PKs (Is there a “natural” PK for a person?) Avoid PKs that change frequently ( ) Phone numbers (not unique, change) All things being equal, choose the simplest candidate key to be the PK Always ensure that your PK defines the entity that you have in mind (Can a SSN change?)

IMS 6217: Primary Key Reference 13 Dr. Lawrence West, MIS Dept., University of Central Florida A Data Model Segment

IMS 6217: Primary Key Reference 14 Dr. Lawrence West, MIS Dept., University of Central Florida Documenting Identifier Attributes (cont.) Identifier attributes are underlined in an ER diagram

IMS 6217: Primary Key Reference 15 Dr. Lawrence West, MIS Dept., University of Central Florida PKs and Other Attributes in SQL Server Diagram Composite PK Single-attribute PK