CS计算机代考程序代写 database Modeling Data in the Organization

Modeling Data in the Organization

Modeling Data in the Organization Data Modeling Part I
© 2013 Pearson Education
1
Modern Database Management
11th Edition, International Edition
Jeffrey A. Hoffer, V. Ramesh,
Heikki Topi

Chapter 2
© 2013 Pearson Education

Objectives
Understand Business Rules
Write good names and definitions for entities, relationships, and attributes
Distinguish unary, binary, and ternary relationships
Model different types of attributes, entities, relationships, and cardinalities
Draw E-R diagrams for common business situations
Convert many-to-many relationships to associative entities
2

Chapter 2
© 2013 Pearson Education

3
Now we begin the journey of learning how to design and use databases. It is exciting to create information systems that run organizations and help people do their jobs well.

The foundation of data models:

Business rules

Chapter 2
© 2013 Pearson Education
Business Rules
Are statements that define or constrain some aspect of the business
Are derived from policies, procedures, events, functions
Assert business structure
Control/influence business behavior
Are expressed in terms familiar to end users
Are automated through DBMS software
4

Chapter 2
© 2013 Pearson Education
Usually within an organization, there will be documentation describes the business procedures or main functions of the business

A Good Business Rule Is:
Declarative–what, not how
Precise–clear, agreed-upon meaning
Atomic–one statement
Consistent–internally and externally
Expressible–structured, natural language
Distinct–non-redundant
Business-oriented–understood by business people
5

Chapter 2
© 2013 Pearson Education

A Good Data Name Is:
Related to business, not technical, characteristics
Meaningful and self-documenting
Unique
Readable
Composed of words from an approved list
Repeatable
Written in standard syntax

6
Data objects must be named and defined before they can be used unambiguously in a model of organizational data.

Chapter 2
© 2013 Pearson Education

E-R Model Constructs
Entities:
Entity instance–person, place, object, event, concept (often corresponds to a row in a table)
Entity Type–collection of entities (often corresponds to a table)
Relationships:
Relationship instance–link between entities (corresponds to primary key-foreign key equivalencies in related tables)
Relationship type–category of relationship…link between entity types
Attributes:
Properties or characteristics of an entity or relationship type (often corresponds to a field in a table)

7

Chapter 2
© 2013 Pearson Education

2

8
Sample E-R Diagram (Figure 2-1)

8
Chapter 2
© 2013 Pearson Education

Chapter 2
© 2013 Pearson Education

3

9
Relationship degrees specify number of entity types involved
Entity symbols

A special entity that is also a relationship

Relationship symbols

Relationship cardinalities specify how many of each entity type is allowed
Attribute symbols

Basic E-R notation (Figure 2-2)
9
Chapter 2
© 2013 Pearson Education

Chapter 2
© 2013 Pearson Education

8

Entities
Entity – a person, a place, an object, an event, or a concept in the user environment about which the organization wishes to maintain data
Entity type – a collection of entities that share common properties or characteristics
Entity instance – A single occurrence of an entity type

10

Chapter 2
© 2013 Pearson Education

Entity Type and Entity Instances
11

Chapter 2
© 2013 Pearson Education

An Entity…
SHOULD BE:
An object that will have many instances in the database
An object that will be composed of multiple attributes
An object that we are trying to model
SHOULD NOT BE:
A user of the database system
An output of the database system (e.g., a report)
12

Chapter 2
© 2013 Pearson Education

13
Inappropriate entities

System
user

System output
Figure 2-4 Example of inappropriate entities
Appropriate entities

Chapter 2
© 2013 Pearson Education
Attributes
Attribute–property or characteristic of an entity or relationship type
Classifications of attributes:
Required versus Optional Attributes
Simple versus Composite Attribute
Single-Valued versus Multivalued Attribute
Stored versus Derived Attributes
Identifier Attributes
14

Chapter 2
© 2013 Pearson Education

5

Required vs. Optional Attributes
15
Required – must have a value for every entity (or relationship) instance with which it is associated
Optional – may not have a value for every entity (or relationship) instance with which it is associated

Chapter 2
© 2013 Pearson Education

5

Simple vs. Composite Attributes
Composite attribute – An attribute that has meaningful component parts (attributes)
16

The address is broken into component parts

Figure 2-7 A composite attribute

Chapter 2
© 2013 Pearson Education

6

17
Figure 2-8 Entity with multivalued attribute (Skill) and derived attribute (Years Employed)
Multivalued
an employee can have more than one skill

Derived
Calculated from date employed and current date
Multi-valued and Derived Attributes
Multivalued – may take on more than one value for a given entity (or relationship) instance
Derived – values can be calculated from related attribute values (not physically stored in the database)
17
Chapter 2
© 2013 Pearson Education

Chapter 2
© 2013 Pearson Education

12

Identifiers (Keys)
Identifier (Key)–an attribute (or combination of attributes) that uniquely identifies individual instances of an entity type
Simple versus Composite Identifier
Candidate Identifier–an attribute that could be a key…satisfies the requirements for being an identifier
18

Chapter 2
© 2013 Pearson Education

6

Criteria for Identifiers
Choose Identifiers that
Will not change in value
Will not be null
Avoid intelligent identifiers (e.g., containing locations or people that might change)
Substitute new, simple keys for long, composite keys
19

Chapter 2
© 2013 Pearson Education

7

20
Figure 2-9 Simple and composite identifier attributes
The identifier is boldfaced and underlined

20
Chapter 2
© 2013 Pearson Education

Chapter 2
© 2013 Pearson Education

14

Naming Attributes
Name should be a singular noun or noun phrase
Name should be unique
Name should follow a standard format
e.g. [Entity type name { [ Qualifier ] } ] Class
Similar attributes of different entity types should use the same qualifiers and classes
21

Chapter 2
© 2013 Pearson Education

7

Modeling Relationships
Relationship Types vs. Relationship Instances
The relationship type is modeled as lines between entity types…the instance is between specific entity instances
Relationships can have attributes
These describe features pertaining to the association between the entities in the relationship
Two entities can have more than one type of relationship between them (multiple relationships)
Associative Entity–combination of relationship and entity
22

Chapter 2
© 2013 Pearson Education

23
Figure 2-10 Relationship types and instances
a) Relationship type (Completes)
b) Relationship instances

23
Chapter 2
© 2013 Pearson Education

Chapter 2
© 2013 Pearson Education

17

Degree of Relationships
Degree of a relationship is the number of entity types that participate in it
Unary Relationship
Binary Relationship
Ternary Relationship
24

Chapter 2
© 2013 Pearson Education

16

25

Degree of relationships – from Figure 2-2
Entities of two different types related to each other

Entities of three different types related to each other

One entity related to another of the same entity type

25
Chapter 2
© 2013 Pearson Education

Chapter 2
© 2013 Pearson Education

8

Cardinality of Relationships
One-to-One
Each entity in the relationship will have exactly one related entity
One-to-Many
An entity on one side of the relationship can have many related entities, but an entity on the other side will have a maximum of one related entity
Many-to-Many
Entities on both sides of the relationship can have many related entities on the other side
26

Chapter 2
© 2013 Pearson Education

27
Figure 2-12 Examples of relationships of different degrees

a) Unary relationships

27
Chapter 2
© 2013 Pearson Education

Chapter 2
© 2013 Pearson Education

22

28
Figure 2-12 Examples of relationships of different degrees (cont.)

b) Binary relationships

28
Chapter 2
© 2013 Pearson Education

Chapter 2
© 2013 Pearson Education

22

29
Figure 2-12 Examples of relationships of different degrees (cont.)

c) Ternary relationship
Note: a relationship can have attributes of its own

29
Chapter 2
© 2013 Pearson Education

Chapter 2
© 2013 Pearson Education

22

Cardinality Constraints
Cardinality Constraints—the number of instances of one entity that can or must be associated with each instance of another entity
Minimum Cardinality
If zero, then optional
If one or more, then mandatory
Maximum Cardinality
The maximum number

30

Chapter 2
© 2013 Pearson Education

29

31
Figure 2-17 Examples of cardinality constraints

a) Mandatory cardinalities

A patient must have recorded at least one history, and can have many

A patient history is recorded for one and only one patient

31
Chapter 2
© 2013 Pearson Education

Chapter 2
© 2013 Pearson Education

1

32
Figure 2-17 Examples of cardinality constraints (cont.)

b) One optional, one mandatory

An employee can be assigned to any number of projects, or may not be assigned to any at all

A project must be assigned to at least one employee, and may be assigned to many

32
Chapter 2
© 2013 Pearson Education

Chapter 2
© 2013 Pearson Education

1

33
Figure 2-17 Examples of cardinality constraints (cont.)

c) Optional cardinalities

A person is married to at most one other person, or may not be married at all

33
Chapter 2
© 2013 Pearson Education

Chapter 2
© 2013 Pearson Education

1

34
Entities can be related to one another in more than one way
Figure 2-21 Examples of multiple relationships

a) Employees and departments

34
Chapter 2
© 2013 Pearson Education

Chapter 2
© 2013 Pearson Education

40

35
Figure 2-21 Examples of multiple relationships (cont.)

b) Professors and courses (fixed lower limit constraint)
Here, min cardinality constraint is 2. At least two professors must be qualified to teach each course. Each professor must be qualified to teach at least one course.

35
Chapter 2
© 2013 Pearson Education

Chapter 2
© 2013 Pearson Education

40

36
Figure 2-15a and 2-15b Multivalued attributes can be represented as relationships
simple
composite
36
Chapter 2
© 2013 Pearson Education

Chapter 2
© 2013 Pearson Education

Strong vs. Weak Entities, and
Identifying Relationships
Strong entity
exists independently of other types of entities
has its own unique identifier
identifier underlined with single line
Weak entity
dependent on a strong entity (identifying owner)…cannot exist on its own
does not have a unique identifier (only a partial identifier)
entity box and partial identifier have double lines
Identifying relationship
links strong entities to weak entities
37

Chapter 2
© 2013 Pearson Education

38
Strong entity
Weak entity
Figure 2-5 Example of a weak identity and its identifying relationship

Click to edit Master text styles
Second level
Third level
Fourth level
Fifth level

Chapter 2
© 2013 Pearson Education

Associative Entities
An entity–has attributes
A relationship–links entities together
When should a relationship with attributes instead be an associative entity?
All relationships for the associative entity should be many
The associative entity could have meaning independent of the other entities
The associative entity preferably has a unique identifier, and should also have other attributes
The associative entity may participate in other relationships other than the entities of the associated relationship
Ternary relationships should be converted to associative entities

39

Chapter 2
© 2013 Pearson Education

40
Figure 2-11a A binary relationship with an attribute
Here, the date completed attribute pertains specifically to the employee’s completion of a course…it is an attribute of the relationship.

40
Chapter 2
© 2013 Pearson Education

Chapter 2
© 2013 Pearson Education

20

41
Figure 2-11b An associative entity (CERTIFICATE)
Associative entity is like a relationship with an attribute, but it is also considered to be an entity in its own right.

Note that the many-to-many cardinality between entities in Figure 2-11a has been replaced by two one-to-many relationships with the associative entity.

41
Chapter 2
© 2013 Pearson Education

Chapter 2
© 2013 Pearson Education

21

42
Figure 2-18 Cardinality constraints in a ternary relationship

42
Chapter 2
© 2013 Pearson Education

Chapter 2
© 2013 Pearson Education

36

43
Figure 2-22
Data model for Pine Valley Furniture Company in Microsoft Visio notation
Different modeling software tools may have different notation for the same constructs.

43
Chapter 2
© 2013 Pearson Education

Chapter 2
© 2013 Pearson Education

44

Copyright © 2013 Pearson Education

Chapter 2
© 2013 Pearson Education

/docProps/thumbnail.jpeg