Upload
devon
View
29
Download
2
Embed Size (px)
DESCRIPTION
Database Systems: Design, Implementation, and Management Eighth Edition. Chapter 4 Entity Relationship (ER) Modeling. Objectives. In this chapter, you will learn: The main characteristics of entity relationship components - PowerPoint PPT Presentation
Citation preview
Database Systems: Design, Implementation, and
ManagementEighth Edition
Chapter 4Entity Relationship (ER) Modeling
Database Systems, 8th Edition 2
Objectives
• In this chapter, you will learn:– The main characteristics of entity relationship
components
– How relationships between entities are defined, refined, and incorporated into the database design process
– How ERD components affect database design and implementation
– That real-world database design often requires the reconciliation (調和 ) of conflicting goals
Database Systems, 8th Edition 3
4.1 The Entity Relationship (ER) Model
• ER model forms the basis of an ER diagram• ERD represents conceptual database as
viewed by end user• ERDs depict database’s main components:
– Entities
– Attributes
– Relationships
Database Systems, 8th Edition 4
Entities
• Refers to entity set and not to single entity occurrence
• Corresponds to table and not to row in relational environment
• In Chen and Crow’s Foot models, entity represented by rectangle with entity’s name
• Entity name, a noun, written in capital letters (大寫 )
Database Systems, 8th Edition 5
Attributes
• Characteristics (特性 ) of entities• Chen notation: attributes represented by ovals
connected to entity rectangle with a line– Each oval contains the name of attribute it
represents
• Crow’s Foot notation: attributes written in attribute box below entity rectangle
Database Systems, 8th Edition 6
Database Systems, 8th Edition 7
Attributes (continued)
• Required (必要 ) attribute: must have a value, represented by bold font (粗體字 )
• Optional (選擇 ) attribute: may be left empty• Domain: set of possible values for an attribute
– Attributes may share a domain
• Identifiers(識別碼 ): one or more attributes that uniquely identify each entity instance
• Composite (組合 ) identifier: primary key composed of more than one attribute
Database Systems, 8th Edition 8
Database Systems, 8th Edition 9
Attributes (continued)
• Composite attribute (組合屬性 ) can be subdivided (分解 )
• Simple attribute cannot be subdivided• Single-value attribute can have only a single
value• Multi-valued attributes (多重數值屬性 ) can
have many values
Database Systems, 8th Edition 10
Database Systems, 8th Edition 11
Attributes (continued)
• M:N relationships and multi-valued attributes should not be implemented
– Create several new attributes for each of the original multi-valued attributes components
– Create new entity composed of original multi-valued attributes components
• Derived (衍生 ) attribute: value may be calculated from other attributes– Need not be physically stored within database
Database Systems, 8th Edition 12
Database Systems, 8th Edition 13
Relationships
• Association (關聯性 ) between entities• Participants (參與者 ) are entities that
participate in a relationship• Relationships between entities always operate
in both directions• Relationship can be classified as 1:M or 1:1• Relationship classification is difficult to establish
if only one side of the relationship is known
Database Systems, 8th Edition 14
Connectivity and Cardinality
• Connectivity (連接性 )– Describes the relationship classification as 1-to-1,
1-to-many, or many-to-many
• Cardinality (基數 )– Expresses minimum and maximum number of
entity occurrences associated with one occurrence of related entity using the format (x, y)
• Established by very concise statements known as business rules
Database Systems, 8th Edition 15
Database Systems, 8th Edition 16
Existence Dependence
• Existence dependence (存在相依 )– Entity exists in database only when it is
associated with another related entity occurrence
• Existence independence (存在獨立 )– Entity can exist apart from one or more related
entities– Sometimes such an entity is referred to as a
strong or regular entity
Database Systems, 8th Edition 17
Relationship Strength
• Weak (non-identifying) relationships (弱關係,非確定性關係 )– Exists if PK of related entity does not contain PK
component of parent entity
• Strong (identifying) relationships (強關係,確定性關係 )– Exists when PK of related entity contains PK
component of parent entity
18Fig 4.8 A weak relationship between COURSE and CLASS
虛線
19Fig 4.9 A strong relationship between COURSE and CLASS
實線
Database Systems, 8th Edition 20
Weak Entities
• Weak entity (弱實體 ) meets two conditions– Existence-dependent
– Primary key partially or totally derived from parent entity in relationship
• Database designer determines whether an entity is weak based on business rules
21Fig 4.10 A weak entity in an ERD
實線
22Fig 4.11 A weak entity in a strong relationship
Database Systems, 8th Edition 23
Relationship Participation
• Optional participation (選擇性參與 )– One entity occurrence does not require
corresponding entity occurrence in particular relationship
• Mandatory participation (強制性參與 )– One entity occurrence requires corresponding
entity occurrence in particular relationship
24
Database Systems, 8th Edition 25
Relationship Degree (關係程度 )
• Indicates number of entities or participants associated with a relationship
• Unary relationship (一元關係 )– Association is maintained within single entity
• Binary relationship (二元關係 )– Two entities are associated
• Ternary relationship (三元關係 )– Three entities are associated
Database Systems, 8th Edition 26
Fig 4.16 The implementation of a ternary relationship27
Database Systems, 8th Edition 28
Recursive (遞迴 ) Relationships
• Relationship can exist between occurrences of the same entity set– Naturally found within unary relationship
Fig 4.20 Implementation of the M:N recursive “PART contains PART” relationship29
Database Systems, 8th Edition 30
Database Systems, 8th Edition 31
Associative (Composite) Entities (關聯實體、組合實體 )
• Also known as bridge entities (橋接實體 )• Used to implement M:N relationships• Composed of primary keys of each of the
entities to be connected• May also contain additional attributes that play
no role in connective process
Database Systems, 8th Edition 32
Database Systems, 8th Edition 33
Database Systems, 8th Edition 34
4.2 Developing an ER Diagram
• Database design is an iterative process (反覆式流程 )
– Create detailed narrative of organization’s description of operations
– Identify business rules based on description of operations– Identify main entities and relationships from business
rules– Develop initial ERD– Identify attributes and primary keys that adequately
describe entities– Revise and review ERD
Database Systems, 8th Edition 35
Database Systems, 8th Edition 36
Database Systems, 8th Edition 37
Database Systems, 8th Edition 38
Database Systems, 8th Edition 39
Database Systems, 8th Edition 40
Fig 4.36 The conceptual UML class diagram for Tiny College 41
42Fig 4.37 The implementation-ready UML class diagram for Tiny College
Database Systems, 8th Edition 43
4.3 Database Design Challenges: Conflicting Goals
• Database designers must make design compromises– Conflicting goals: design standards, processing
speed, information requirements• Important to meet logical requirements and
design conventions (慣例 )• Design of little value unless it delivers all
specified query and reporting requirements• Some design and implementation problems do
not yield “clean” solutions
44Fig 4.38 Various Implementation of the 1:1 recursive relationship
Database Systems, 8th Edition 45
Summary
• Entity relationship (ER) model – Uses ERD to represent conceptual database as
viewed by end user
– ERM’s main components:• Entities• Relationships• Attributes
– Includes connectivity and cardinality notations
Database Systems, 8th Edition 46
Summary (continued)
• Connectivities and cardinalities are based on business rules
• M:N relationship is valid at conceptual level– Must be mapped to a set of 1:M relationships
• ERDs may be based on many different ERMs• Database designers are often forced to make
design compromises