The degree of a relationship type is the number of

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The degree of a relationship type is the number of participating entity types. Both MANAGES and WORKS_ON are binary relationships. The Entity-Relationship Model 23
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Relationship instances of the WORKS_FOR N:1 relationship between EMPLOYEE and DEPARTMENT The Entity-Relationship Model 24
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Relationship instances of the M:N WORKS_ON relationship between EMPLOYEE and PROJECT The Entity-Relationship Model 25
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Relationship Type vs. Relationship Set Relationship Type: Is the schema description of a relationship Identifies the relationship name and the participating entity types Also identifies certain relationship constraints Relationship Set: The current set of relationship instances represented in the database The current state of a relationship type The Entity-Relationship Model 26
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Relationship Type vs. Relationship Set By examining the requirements, six relationship types are identified All are binary relationships( degree 2) Listed below with their participating entity types: WORKS_FOR (between EMPLOYEE, DEPARTMENT) MANAGES (also between EMPLOYEE, DEPARTMENT) CONTROLS (between DEPARTMENT, PROJECT) WORKS_ON (between EMPLOYEE, PROJECT) SUPERVISION (between EMPLOYEE (as subordinate), EMPLOYEE (as supervisor)) DEPENDENTS_OF (between EMPLOYEE, DEPENDENT) The Entity-Relationship Model 27
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ER DIAGRAM – Relationship Types are: WORKS_FOR, MANAGES, WORKS_ON, CONTROLS, SUPERVISION, DEPENDENTS_OF The Entity-Relationship Model 28
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Discussion on Relationship Types In the refined design, some attributes from the initial entity types are refined into relationships: Manager of DEPARTMENT -> MANAGES Works_on of EMPLOYEE -> WORKS_ON Department of EMPLOYEE -> WORKS_FOR Etc. In general, more than one relationship type can exist between the same participating entity types MANAGES and WORKS_FOR are distinct relationship types between EMPLOYEE and DEPARTMENT Different meanings and different relationship instances. The Entity-Relationship Model 29
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Constraints on Relationships Constraints on Relationship Types (Also known as ratio constraints) Cardinality Ratio (specifies maximum participation) One-to-one (1:1) One-to-many (1:N) or Many-to-one (N:1) Many-to-many (M:N) Existence Dependency Constraint (specifies minimum participation) (also called participation constraint) zero (optional participation, not existence-dependent) one or more (mandatory participation, existence-dependent) The Entity-Relationship Model 30
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Many-to-one (N:1) Relationship The Entity-Relationship Model 31
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Many-to-many (M:N) Relationship The Entity-Relationship Model 32
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Recursive Relationship Type A relationship type between the same participating entity type in distinct roles Also called a self-referencing relationship type.
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  • Fall '09
  • SUNANHAN
  • Entity-relationship model, ER Diagrams

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