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Entity Types Representing
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(M- Way)
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S o m e E R D notations support relationships involving more than two entity types known as M - w a y (multiway) relationships where the M m e a n s more than two For example, the Chen
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Understanding Entity Relationship Diagrams 145
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FIGURE 515 M-Way (Ternary) Relationship Using the Chen Notation
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Part PartNo PartName
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Supplier SuppNo SuppName
Project ProjNo ProjName
FIGURE 516 Associative Entity Type to Represent a Ternary Relationship
Part PartNo PartName
Supplier SuppNo SuppName Supposes
Project ProjNo ProjName
Part-Uses Associativeentity type
Uses
Proj-Uses
E R D notation (with diamonds for relationships) allows relationships to connect more than two entity types, as depicted in Figure 5 1 5 The Uses relationship lists suppliers and parts used on projects For example, a relationship instance involving Supplierl, Parti, and Projectl indicates that Supplierl Supplies Parti on Projectl A n M - w a y relationship involving three entity types is called a ternary relationship Although y o u cannot directly represent M - w a y relationships in the Crow's Foot nota associative entity t y p e tion, y o u should understand h o w to indirectly represent them You use an associative entity type and a collection o f identifying 1 -M relationships to represent an M-way relationship In Figure 516, three 1 -M relationships link the associative entity type, Uses, to the Part, the Supplier, and the Project entity types The Uses entity type is associative because its role is to connect other entity types Because associative entity types provide a connecting role, they are sometimes given names using active verbs In addition, associative entity types are always weak as they must borrow the entire primary key For example, the Uses entity type obtains its primary key through the three identifying relationships A s another example, Figure 517 shows the associative entity type Provides nects the Employee, Skill, and Project entity types A n example instance o f the entity type contains E m p l o y e e l providing Skilll on Projectl that con Provides
a weak entity that depends on two or more entity types for its primary key An associative entity type with more than two identifying relationships is known as an M-way associative entity type
The Chen notation is named after Dr Peter Chen, who published the paper defining the Entity Relationship Model in 1976
Part Three
Data Modeling
FIGURE 5 1 7 Associative Entity Type Connecting
Employee, Skill, and Project
Employee EmpNo EmpName
Skill SkilINo SkillName Skill-Uses
Project ProjNo ProjName
Emp-Uses
Provides
Proj-Uses
FIGURE 5 1 8 Enrollsln M-N
Relationship (Figure 510) Transformed into 1-M Relationships
Student StdSSN StdName
Offering OfferNo OffLocation
Registers
Enrollment EnrGrade
Grants
The issue o f when to use an M-way associative entity type (ie, an associative entity type representing an M-way relationship) can be difficult to understand If a database only needs to record pairs o f facts, an M-way associative entity type is not needed For example, if a database only needs to record w h o supplies a part and what projects use a part, then an M-way associative entity type should not be used In this case, there should be binary relationships between Supplier and Part and between Project and Part You should use an M - w a y associative entity type w h e n the database should record combinations o f three (or more) entities rather than just combinations o f two entities For example, if a database needs to record which supplier provides parts o n specific projects, an M-way associative entity type is needed Because o f the complexity o f M - w a y relationships, 7 provides a way to reason about them using constraints, while 12 provides a way to
relationship equivalence
reason about them using data entry forms
a M-N relationship can be replaced by an associative entity type and two identifying 1-M relationships In most cases the choice between a M-N relationship and the associative entity type is personal preference
Equivalence b e t w e e n 1-M a n d M - N Relationships
To improve your understanding o f M - N relationships, y o u should know an important equivalence for M - N relationships A n M - N relationship can be replaced by an associative entity type and two identifying 1-M relationships Figure 518 shows the Enrollsln ships and an associative entity type replace the Enrollsln name (Enrollsln) has been changed to a noun (Enrollment) (Fig ure 510) relationship converted to this 1-M style In Figure 518, two identifying relation relationship The relationship to follow the convention o f
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