Chapter 4 - The Semantic Object Model

first developed in 1988 by Codd, Hammer and McLeod

Semantic Objects

"purpose of a database application is to provide forms, reports and queries so that the user can record data and obtain the information they need about entities or objects important to their work"
"main goals of the early stages of database development are to build a data model that documents the things to be represented in the database, to determine the characteristics of those those things that need to be stored, and to establish the relationships among them"

Defining Semantic Objects
- A semantic object is a named collection of attributes that sufficiently describe a distinct identity (representation of some identifiable thing in the users' work environment
- Attributes respresent all of the characteristics of the identifier being represented
- Identities are the nouns about which the information is to be produced
- Identities that the object represent may or may not have a physical representation

Attributes

simple attributes have a single value
group attributes are composites of other attributes
semantic object attributes establish a relationship between semantic objects

semantic object diagram (object diagram)

_IDDEPARTMENT_1.1

DepartmentName_1.1

CampusAddress

Building_1.1
OfficeNumber_1.1

_0.1

PhoneNumber_1.N

FaxPhoneNumber_0.1

COLLEGE_1.1

PROFESSOR_1.N

STUDENT_1.N

object links (object attributes)

ATTRIBUTE CARDINALITY (minimum and maximum)
OBJECT INSTANCES
PAIRED ATTRIBUTES (ex. DEPARTMENT contains COLLEGE & COLLEGE contains DEPARTMENT

Object Identifiers - one or more object attributes that are used to identify object instances (i.e. StudentName, StudentID)
group identifier (identifier has more than one attribute)
Attribute Domains - description of an attribute's possible values [may be an enumerated list
Semantic Object Views - portion of an object that is visible to a particular application [can be used to develop the data model]

Creating Data Models with Semantic Objects
- reverse engineer from reports, forms, and queries
- start from scratch

An Example: The Highline University Administration Database

THE COLLEGE OBJECT

COLLEGE

_ID CollegeName_1.1

  DeanName_1.1

  PhoneNumber_1.N

CampusAddress

Building_1.1
OfficeNumber_1.1
_0.1

  DEPARTMENT_1.N

THE DEPARTMENT OBJECT

DEPARTMENT

_ID DeptName_1.1

ChairPerson_1.1

PhoneNumber_1.N

TotalMajors_1.1

CampusAddress

Building_1.1
OfficeNumber_1.1

_0.1

COLLEGE_1.1

PROFESSOR_1.N

STUDENT_1.N

THE PROFESSOR OBJECT

PROFESSOR

_ID ProfessorName_1.1

PhoneNumber_1.N

CampusAddress

Building_1.1
OfficeNumber_1.1
_0.1

DEPARTMENT_1.1
THE STUDENT OBJECT

STUDENT

_ID StudentName_1.1

_ID StudentNumber_1.1

PhoneNumber_1.N

DEPARTMENT_1.1

Specifying Objects

Semantic Object Specification
Object Name	Property Name	Min Card	Max Card	ID Status	Domain Name
COLLEGE	CollegeName	1	1	ID	CollegeName
	DeanName	1	1		PersonName
	FirstName	0	1		FirstName
	LastName	1	1		LastName
	PhoneNumber	0	1		Phone
	CampusAddress	0	1		CampusAddress
	Building	1	1		Building
	OfficeNumber	1	1		OfficeNumber
	DEPARTMENT	1	N		DEPARTMENT

DEPARTMENT

PROFESSOR

STUDENT

Domain Specifications
Domain Name Type Semantic Description Physical Description

Address Group A U.S. Address Street, City, State, Zip

Building Simple A name of a building on campus Text 20

Campus Address G An address on campus Building, Office Number

City S A city name Text 25

COLLEGE Semantic Object One of Highline's ten colleges See semantic object specification table

CollegeName S The official name of a college at Highline Text 25

DEPARTMENT SO An academic department on campus See semantic object specification table

Types of Objects
- Simple Objects - contains only single-valued, nonobject attributes (ex. EQUIPMENT)
- Composite Objects - contains one or more multivalued nonobject attributes (ex. HOTEL-BILL)
- Compound Objects - contains at least one object attribute (ex. COLLEGE)
  Four Types of Compound Objects
  Object2
  Can
  Contain Object1 Can Contain
  
  One Many
  
  One 1:1 1:N
  
  Many M:1 M:N
- Hybrid Objects - combinations of objects of two types (semantic object with at least one multivalued group attribute that includes a semantic object attribute
  DORMITORY Hybrid Object
  DORMITORY
  
  _ID  DormName
  
       ResidentAssistant
  
       Phone
  
       StudentRent
  
       STUDENT_1.1
  
         Rent_0.1
  _1.N
- Association Objects - object that relates two (or more) objects and stores data that are peculiar to that relationship [ex. FLIGHT is an association object that associates AIRPLANE and PILOT objects
- Parent/Subtype Objects - the subtype inherits all of the attributes of its parent [ex. MANAGER / EMPLOYEE where a manager is also an employee]
- Archtype/Version Objects - produces other semantic objects that represent versions, releases, or editions of the archetype [ex. TEXTBOOK produces EDITIONs]
Comparing the Semantic Object and the E-R Model
- Similarities
  1. both are tools for understanding and documenting the structure of the users' data
  2. both strive to model the structure of the things in the users' world and the relationships among them
- Differences
  1. principal difference is one of orientation
  2. E-R model: concept of entity is basic [entities and relationships are pieces of the data model]
  3. S-O model: concept of semantic object is basic[set of semantic objects in a data model is a map of the essential structure of the things that the user considers important]
  4. semantic objects are semantically self-contained (contains metadata about the "objects")

Domain Name	Type	Semantic Description	Physical Description
Address	Group	A U.S. Address	Street, City, State, Zip
Building	Simple	A name of a building on campus	Text 20
Campus Address	G	An address on campus	Building, Office Number
City	S	A city name	Text 25
COLLEGE	Semantic Object	One of Highline's ten colleges	See semantic object specification table
CollegeName	S	The official name of a college at Highline	Text 25
DEPARTMENT	SO	An academic department on campus	See semantic object specification table

Four Types of Compound Objects
Object1 Can Contain
	One	Many
One	1:1	1:N
Many	M:1	M:N