Ch 05 . Basic Symbols (manino) Entity Type symbol Relationship symbol Course Primary Key Attributes CourseNo CrsDesc CrsUnits Entity Type name Offering Has Relationship name OfferNo OffLocation OffTime Cardinalities Course Offering Course1 Offering1 Course2 Offering2 Course3 Offering3 Offering4 Cardinality Notation Perpendicular line: one cardinality Crow's foot: many cardinality Inside symbol: minimum cardinality Course Offering CourseNo CrsDesc CrsUnits OfferNo OffLocation OffTime Has Outside symbol: maximum cardinality Circle: zero cardinality Classification of Cardinalities Minimum cardinality based – Mandatory: existence dependent – Optional Maximum cardinality based – – – – Functional 1-M M-N 1-1 Summary of Cardinalities Classification Mandatory Cardinality Restrictions Minimum cardinality 1 Optional Minimum cardinality = 0 Functional or single- Maximum cardinality = 1 valued 1-M Maximum cardinality = 1 in one direction and Maximum cardinality > 1 in the other direction. M-N Maximum cardinality is > 1 in both directions. 1-1 Maximum cardinality = 1 in both directions. More Relationship Examples Faculty Office OfficeNo OffPhone OffType WorksIn FacSSN FacSalary FacRank FacHireDate Offering TeamTeaches OfferNo OffLocation OffTime Comparison to Access Notation CourseNo CrsDesc CrsUnits ERD (Crow's Foot) Offering OfferNo OffLocation OffTime Has Access Relationship Diagram Offering OfferNo Course 1 CourseNo Courseno OffLocation CrsDesc OffTime CrsUnits ... 8 Course Understanding Relationships Identification dependency M-N relationships with attributes Self identifying relationships M-way relationships Equivalence between M-N and 1-M relationships Identification Dependency Identification Dependency Symbols: Solid relationship line for identifying relationships Diagonal lines in the corners denote weak entities. Building BldgID BldgName BldgLocation Room Contains RoomNo RoomCapacity M-N Relationships with Attributes Offering Student StdSSN StdName EnrollsIn EnrGrade attribute of relationship OfferNo OffLocation OffTime M-N Relationships with Attributes (II) b) Writes relationship Author Book AuthNo AuthName Writes ISBN Title AuthOrder a) Provides relationship Supplier SuppNo SuppName Part Provides Qty PartNo PartName Instance Diagrams for SelfReferencing Relationships (a) Supervises (b) PreReqTo Faculty1 IS300 IS320 Faculty2 Faculty3 IS480 Faculty4 IS460 Faculty5 IS461 ERD Notation for SelfReferencing Relationships a) manager-subordinate Faculty FacSSN FacName Supervises b) course prerequisites Course CourseNo CrsDesc PrereqTo Associative Entity Types for M-way Relationships Part PartNo PartName Supplier Project SuppNo SuppName ProjNo ProjName Supp-Uses Part-Uses Associative entity type Uses Proj-Uses Relationship Equivalence Replace M-N relationship – Associative entity type – Two identifying 1-M relationships M-N relationship versus associative entity type – Largely preference – Associative entity type is more flexible in some situations Associative Entity Type Example Offering Student OfferNo OffLocation OffTime StdSSN StdName Registers Enrollment EnrGrade Grants Attendance RecordedFor AttDate Present Generalization Hierarchies generalization hierarchy symbol SalaryEmp EmpSalary Employee EmpNo EmpName EmpHireDate ... subtypes supertype HourlyEmp EmpRate Inheritance Subtypes inherit attributes of supertypes (direct and indirect) Allows abbreviation of attribute list Applies to code (methods) as well as attributes (data) Generalization Constraints Disjointness Constraint Security Symbol SecName LastClose Completeness Constraint D,C Stock OutShares IssuedShares Bond Rate FaceValue Multiple Levels of Generalization Security Symbol SecName LastClose D,C Stock OutShares IssuedShares D,C Common PERatio Dividend Preferred CallPrice Arrears Bond Rate FaceValue Comprehensive Example UnivPerson SSN Name City State Zip C Student Offering StdClass StdMajor StdGPA OfferNo OffLocation OffTime Faculty Teaches Has FacSalary FacRank FacHireHate Supervises Registers Grants Enrollment EnrGrade Course CourseNo CrsDesc CrsUnits Diagram Rules Ensure that ERD notation is correctly used Similar to syntax rules for a computer language Completeness rules: no missing specifications Consistency rules: no conflicts among specifications Supported by the ER Assistant Completeness Rules Primary Key Rule: all entity types have a PK (direct, indirect, or inherited) Naming Rule: all entity types, relationships, and attributes have a name Cardinality Rule: cardinality is specified in both directions for each relationship Entity Participation Rule: all entity types participate in an at least one relationship except for entity types in a generalization hierarchy Generalization Hierarchy Participation Rule: at least one entity type in a generalization hierarchy participates in a relationship Primary Key Rule Issue Primary key rule is simple in most cases For some weak entities, the PK rule is subtle – Weak entity with only one 1-M identifying relationship – Weak entity must have a local key to augment the borrowed PK from the parent entity type – Violation of PK rule if local key is missing PK Rule Violation Example PK rule violation A single 1-M identifying relationship Room does not have a local key. Building BldgID BldgName BldgLocation Room Contains RoomNo RoomCapacity Naming Consistency Rules Entity Name Rule: entity type names must be unique Attribute Name Rule: attribute names must be unique within each entity type and relationship Inherited Attribute Rule: attribute names in a subtype do not match inherited (direct or indirect) attribute names. Connection Consistency Rules Relationship/Entity Connection Rule: relationships connect two entity types (not necessarily distinct) Relationship/Relationship Connection Rule: relationships are not connected to other relationships Redundant Foreign Key Rule: foreign keys are not used. Identification Dependency Rules Weak entity rule: weak entities have at least one identifying relationship Identifying relationship rule: at least one participating entity type must be weak for each identifying relationship Identification dependency cardinality rule: the minimum and maximum cardinality must equal 1 for a weak entity in all identifying relationships Example of Diagram Errors UnivPerson SSN Name City State Zip Rule 6 Violation (Weak Entity) C Student StdClass StdMajor StdGPA Offering OfferNo OffLocation Rule 9 Violation OffTime (Redundant FK) CourseNo Rule 8 Violation (Id Dependency Cardinality) Registers Enrollment EnrGrade Faculty Teaches Has Rule 7 Violation (Identifying Relationship) Grants Course CourseNo CrsDesc CrsUnits FacSalary FacRank FacHireHate Supervises Corrected ERD UnivPerson SSN Name City State Zip C Student Offering StdClass StdMajor StdGPA OfferNo OffLocation OffTime Faculty Teaches Has FacSalary FacRank FacHireHate Supervises Registers Grants Enrollment EnrGrade Course CourseNo CrsDesc CrsUnits
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