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

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

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

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

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)

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

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

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

“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

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

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”

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

A Data Model Segment

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

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