database systems: design, implementation, and management tenth edition
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Database Systems: Design, Implementation, and Management Tenth Edition. Chapter 4 Entity Relationship (ER) Modeling. Connectivity and Cardinality. Connectivity Describes the relationship classification Cardinality - PowerPoint PPT PresentationTRANSCRIPT
Database Systems: Design, Implementation, and
ManagementTenth Edition
Chapter 4Entity Relationship (ER) Modeling
Connectivity and Cardinality
• Connectivity – Describes the relationship classification
• Cardinality – Expresses minimum and maximum number of
entity occurrences associated with one occurrence of related entity
• Established by very concise statements known as business rules
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Derived Attributes
• Derived attribute: value may be calculated from other attributes– Need not be physically stored within database
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Existence Dependence
• Existence dependence– Entity exists in database only when it is
associated with another related entity occurrence
• Existence independence– Entity can exist apart from one or more related
entities
– Sometimes such an entity is referred to as a strong or regular entity
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Relationship Strength
• Weak (non-identifying) relationships– Exists if PK of related entity does not contain PK
component of parent entity
• Strong (identifying) relationships– Exists when PK of related entity contains PK
component of parent entity
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Weak Entities
• Weak entity 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
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Recursive Relationships
• Relationship can exist between occurrences of the same entity set
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Recursive Relationships• Relationship can exist between occurrences of
the same entity set
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Recursive Relationships
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1:1 Recursive Relationship Implementation• EMPLOYEE_V1 table can yield anomalies if Anne Jones
divorces Anton Shapiro– Two records must be updated, if only one is updated then we
have inconsistent data
– Nulls for those employees not married to other employees
• MARRIED_V1 eliminates nulls for employees not married to other employees but duplicate values are still possible (345,347) and (347,345)
• Third method requires collection of marriage date. Unless sending out congratulations each year, the information is unnecessary
• Theoretically possible to have more than two people in a marriage
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Developing an ER Diagram
• Database design is an iterative process– 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
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Design Challenges• Must conform to design standards
– Minimize redundancies and avoid nulls when possible
• Processing Speed– High processing speeds are a top priority in today’s online world
– Combine two tables into one which may be less elegant but faster data retrieval
– Include derived attributes instead of computing them as needed
• Information Requirements– Complex information requirements might require additional
entities and attributes within the design
– May be worth sacrificing “clean” design and speed to ensure maximum information generation
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Summary (cont’d.)
• 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• UML class diagrams are used to represent the
static data structures in a data model• Database designers are often forced to make
design compromises
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