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B e c a u s e the identification dependency rules and the redundant foreign key rule are a frequent source o f errors to novice designers, this section provides an example to depict rule violations and resolutions Figure 525 demonstrates violations o f the identification dependency rules (consistency rules 6 to 9) and the redundant foreign k e y rule (consistency rule 9) for the university database ERD The following list explains the violations: Consistency rule 6 (weak entity rule) violation: Faculty cannot be a weak entity without at least one identifying relationship
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Consistency rule 7 (identifying relationship rule) violation: The Has relationship is
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identifying but neither Offering nor Course is a weak entity
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Consistency rule 8 (identification dependency cardinality rule) violation: The
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cardinality o f the Registers relationship from Enrollment to Student should be ( 1 , 1) not (0, Many)
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Consistency rule 9 (redundant foreign key rule) violation: The CourseNo attribute in
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the Offering entity type is redundant with the Has relationship Because CourseNo is the primary key o f Course, it should not be an attribute o f Offering to link an Offering to a Course The Has relationship provides the linkage to Course
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UnivPerson SSN Name City State Zip Rule 6 violation (weak entity) Student StdClass StdMajor StdGPA Offering OfferNo OffLocation OffTime CourseNo X)
Faculty Teaches O Has Supervises FacSalary FacRank FacHireDate
Rule 9 violation (redundant FK) Rule 8 violation (ID dependency
Rule 7 violation (identifying relationship) Grants Course CourseNo CrsDesc CrsUnits
Registers
' 1 I 1 1
Enrollment EnrGrade
For most rules, resolving violations is easy The major task is recognition o f the viola tion For the identification dependency rules, resolution can depend o n the problem details The following list suggests possible corrective actions for diagram errors: Consistency rule 6 (weak entity rule) resolution: The problem can be resolved b y either adding one or more identifying relationships or by changing the weak entity into a regular entity In Figure 525, the problem is resolved by making Faculty a regular en tity The more c o m m o n resolution is to add one or more identifying relationships
Consistency rule 7 (identifying relationship rule) resolution: The problem can be
resolved by adding a weak entity or making the relationship nonidentifying In Fig ure 525, the problem is resolved by making the Has relationship nonidentifying If there is more than one identifying relationship involving the same entity type, the typi cal resolution involves designating the c o m m o n entity type as a weak entity
Consistency rule 8 (identification dependency cardinality rule) resolution: The
problem can be resolved by changing the weak entity's cardinality to (1,1) Typically, the cardinality o f the identifying relationship is reversed In Figure 525, the cardinality o f the Registers Enrollment) relationship should be reversed ((1,1) near Student and (0, Many) near
Consistency rule 9 (redundant foreign key rule) resolution: Normally the problem can be resolved by removing the redundant foreign key In Figure 525, should be removed as an attribute o f Offering renamed instead o f removed CourseNo In s o m e cases, the attribute may not
represent a foreign key If the attribute does not represent a foreign key, it should be
5
Understanding Entity Relationship Diagrams
TABLE 55 Alternative Rule Organization
Category Names
Rules All entity types, relationships, and attributes are named (Completeness rule 2) Entity type names are unique (Consistency rule 1) Attribute names are unique within entity types and relationships (Consistency rule 2) Attribute names in a subtype do not match inherited (direct or indirect) attribute names (Consistency rule 3) All entity types have a primary key (direct, borrowed, or inherited) (Completeness rule 1) Cardinality is given for both entity types in a relationship (Completeness rule 3) All entity types except those in a generalization hierarchy participate in at least one relationship (Completeness rule 4) Each generalization hierarchy participates in at least one relationship with an entity type not in the generalization hierarchy (Completeness rule 5) All relationships connect two entity types (Consistency rule 4) Relationships are not connected to other relationships (Consistency rule 5) Redundant foreign keys are not used (Consistency rule 9) Weak entities have at least one identifying relationship (Consistency rule 6) For each identifying relationship, at least one participating entity type must be weak (Consistency rule 7) For each weak entity, the minimum and maximum cardinality must equal 1 for each identifying relationship (Consistency rule 8)
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