Chapter 17 - Designing the File or Database

1. Elements of the Entity-Relationship Model
   Introduced by Peter Chen in 1976
   • Entities
     entity - something that can be identified in the usrs' work environment (something that the user wants to track); i.e. EMPLOYEE Mary Doe, CUSTOMER 12345, SALES-ORDER 1000, etc.
     entity class - groups of entities of a given type; i.e. EMPLOYEE
   • Attributes
     attribute (property) - describes a characteristic of an entity; i.e. EmployeeName, DateOfHire, JobSkillCode
     attributes can be single or multiple valued (ie. courses taken); can also be composite (i.e. Address)
   • Indentifiers
     identifier attributes that name the entity instance (i.e. EmployeeID, EmployeeName)
     identifiers can be unique or nonunique
     composite identifiers consist of more than one attribute (i.e. [FirstName, LastName, PhoneExtension])
   • Relationships
     relationship - association between entities
     degree of a relationship - number of entities in the relationship (most relationships are binary relationships
     THREE TYPE OF BINARY RELATIONSHIPS:
     • 1:1 - STUDENT - HIGH-SCHOOL-GRADUATED-FROM
     • 1:N (1 to many) - DEPARTMENT - FACULTY
     • N:M (many to many) - BOOK - AUTHOR
     
     maximum cardinality - maximum number of elements that occur on one side of a relationship
     These are examples of HAS-A relationships
     ENTITY-RELATIONSHIP DIAGRAMS
     
     minimum cardinality:
     • Use oval (0) to indicate that an entity is not required
     • Use hash (|) to indicate that an entity must exist
     • Use crow's feet (/) to indicate that an entity may have many instances

     File Types

     • Master file
     • Table file
     • Transaction file
     • Work file
     • Report file

     File Organization

     • Sequential
     • Linked Lists
     • Hashed (Direct access / relative)
     • Indexed (requires file of indicies)
     • Indexed Sequential (ISAM, VSAM)
   • Normalization
   • Guidelines for File/Database Relation Design
     • Each separate data entity should create a master file
     • A specific data field should exist in only one master file
     • Each master file / database relation should have programs to Create, Read, Update, and Delete records
   • Making Use of the Database
     1. Choose a relation from the database
     2. Join two relations together
     3. Project columns from the relation
     4. Select rows from the relation
     5. Derive new attributes
     6. Index / sort rows
     7. Calculate totals and performance measures
     8. Present results

Exercises: (due - noon Nov. 24, 1998)
e-mail to summers_wayne@ColumbusState.edu your answers to problem #7 on page 649.