DATABASE SYSTEMS Chapter 5 Entity Relationship (ER) Modelling
Transcript of DATABASE SYSTEMS Chapter 5 Entity Relationship (ER) Modelling
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DATABASE SYSTEMS
DESIGN IMPLEMENTATION AND MANAGEMENT
INTERNATIONAL EDITION
ROB • CORONEL • CROCKETT
Chapter 5 Entity Relationship (ER) Modelling
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In this chapter, you will learn:
• The main characteristics of entity relationship components
• How relationships between entities are defined and refined and how those relationships are incorporated into the database design process
• How ERD components affect database design and implementation
• That real-world database design often requires the reconciliation of conflicting goals
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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
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A Recipe ERD
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Entities
• Refers to entity set and not to single entity occurrence
• Corresponds to table and not to row in relational environment
• In UML notation, an entity is represented by a box that is subdivided into three parts.
• Entity name, a noun, is usually written in capital letters
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Attributes
• In the UML model, the attributes are written in the attribute box below the entity rectangle
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Domains
• Attributes have domain – Domain is attribute’s set of possible values
• Attributes may share a domain
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Identifiers (Primary Keys)
• Underlined in the ERD
• Key attributes are also underlined in frequently used table structure shorthand
• For example, a CAR entity may be represented by:
• CAR (CAR_REG, MOD_CODE, CAR_YEAR, CAR_COLOUR)
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Composite Primary Keys
• Primary keys ideally composed of only single attribute
• Possible to use a composite key – Primary key composed of more than one attribute
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Composite Primary Keys (continued)
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Composite and Simple Attributes
• Composite attribute can be subdivided
• Simple attribute cannot be subdivided
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Single and Multi-Valued Attributes
• Single-value attribute can have only a single value
• Multi-valued attributes can have many values
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Multivalued Attributes (continued)
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Resolving Multivalued Attribute Problems
• Although conceptual model can handle M:N relationships and multi-valued attributes, you should not implement them in relational DBMS – Within original entity, create several new attributes, one for
each of the original multi-valued attribute’s components • Can lead to major structural problems in table
– Create new entity composed of original multivalued attribute’s components
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Resolving Multivalued Attribute Problems (continued)
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Resolving Multivalued Attribute Problems (continued)
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Resolving Multivalued Attribute Problems (continued)
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Derived Attributes
• Attribute whose value may be calculated (derived) from other attributes
• Need not be physically stored within database
• Can be derived by using an algorithm
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Derived Attributes (continued)
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Derived Attributes (continued)
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Relationships
• Association between entities
• Participants are entities that participate in a relationship
• Relationships between entities always operate in both directions
• Relationship can be classified as 1:*
• Relationship classification is difficult to establish if know only one side of the relationship
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Multiplicity
• Multiplicity refers to the number of instances of one entity that are associated with one instance of a related entity.
• Cardinality expresses minimum and maximum number of entity occurrences associated with one occurrence of related entity
• Participation - determines whether all occurrences of an entity participate in the relationship or not.
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Multiplicity(continued)
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Existence Dependence
• Existence dependence – Exist 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 refers to such an entity as 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 (Non-Identifying) Relationships
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Weak (Non-Identifying) Relationships (continued)
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Weak (Non-Identifying) Relationships (continued)
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Strong (Identifying) Relationships
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Weak Entities
• Weak entity meets two conditions – Existence-dependent
• Cannot exist without entity with which it has a relationship
– Has primary key that is partially or totally derived from parent entity in relationship
• Database designer usually determines whether an entity can be described as weak based on business rules
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Weak Entities (continued)
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Weak Entities (continued)
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Relationship Participation
• Optional participation – One entity occurrence does not require
corresponding entity occurrence in particular relationship
• Mandatory participation – One entity occurrence requires corresponding entity
occurrence in particular relationship
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Relationship Participation (continued)
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Relationship Participation (continued)
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Relationship Participation (continued)
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Relationship Participation (continued)
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Relationship Degree
• Indicates number of entities or participants associated with a relationship
• Unary relationship – Association is maintained within single entity
• Binary relationship – Two entities are associated
• Ternary relationship – Three entities are associated
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Relationship Degree (continued)
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Relationship Degree (continued)
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Relationship Degree (continued)
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Recursive Relationships
• Relationship can exist between occurrences of the same entity set
• Naturally found within unary relationship
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Recursive Relationships (continued)
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Recursive Relationships (continued)
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Recursive Relationships (continued)
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Recursive Relationships (continued)
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Recursive Relationships (continued)
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Recursive Relationships (continued)
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Composite Entities
• Also known as bridge entities
• Composed of primary keys of each of the entities to be connected
• May also contain additional attributes that play no role in connective process
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Composite Entities (continued)
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Composite Entities (continued)
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Composite Entities (continued)
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Association Class
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Developing an ER Diagram
• Database design is iterative rather than linear or sequential process
• Iterative process – Based on repetition of processes and procedures
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Developing an ER Diagram (continued)
• Building an ERD usually involves the following activities: – 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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Developing an ER Diagram (continued) • Tiny University
– Tiny University is divided into several schools • Each school is composed of several departments
– Each department may offer courses – Each department may have many lecturers assigned to it – Each lecturer may teach up to four classes; each class is
section of course – Student may enroll in several classes, but (s)he takes each
class only once during any given enrollment period
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Developing an ER Diagram (continued)
• Tiny University (continued) – Each department has several students
• Each student has only a single major and is associated with a single department
– Each student has an advisor in his or her department • Each advisor counsels several students
– The relationship between class is taught in a room and the room in the building
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Developing an ER Diagram (continued)
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Developing an ER Diagram (continued)
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Developing an ER Diagram (continued)
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Developing an ER Diagram (continued)
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Developing an ER Diagram (continued)
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Developing an ER Diagram (continued)
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Developing an ER Diagram (continued)
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Developing an ER Diagram (continued)
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Developing an ER Diagram (continued)
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Developing an ER Diagram (continued)
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Developing an ER Diagram (continued)
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Database Design Challenges: Conflicting Goals
• Database design must conform to design standards
• High processing speeds are often a top priority in database design
• Quest for timely information might be focus of database design
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Database Design Challenges: Conflicting Goals (cont.)
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Database Design Challenges: Conflicting Goals (cont.)
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Summary
• Entity relationship (ER) model – Uses ERD to represent conceptual database as viewed
by end user – ERM’s main components:
• Entities • Relationships • Attributes
– Includes multiplicity notation.
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Summary (continued)
• Multiplicities are based on business rules
• In ERM, *:* relationship is valid at conceptual level
• ERDs may be based on many different ERMs
• Database designers are often forced to make design compromises