CAS第二次课程实体关系模型.ppt

Chapter 2:Entity-Relationship Model,Entity SetsRelationship SetsDesign Issues Mapping Constraints KeysE-R DiagramExtended E-R FeaturesDesign of an E-R Database SchemaReduction of an E-R Schema to Tables,Entity Sets,A database can be modeled as:a collection of entities,relationship among entities.An entity is an object that exists and is distinguishable from other objects.Example:specific person,company,event,plantEntities have attributesExample:people have names and addressesAn entity set is a set of entities of the same type that share the same properties.Example:set of all persons,companies,trees,holidays,Entity Sets customer and loan,customer-id customer-customer-customer-loan-amount name street city number,Attributes,An entity is represented by a set of attributes,that is descriptive properties possessed by all members of an entity set.Domain the set of permitted values for each attribute Attribute types:Simple and composite attributes.Single-valued and multi-valued attributesE.g.multivalued attribute:phone-numbersDerived attributesCan be computed from other attributesE.g.age,given date of birth,Example:customer=(customer-id,customer-name,customer-street,customer-city)loan=(loan-number,amount),Composite Attributes,Relationship Sets,A relationship is an association among several entitiesExample:HayesdepositorA-102customer entityrelationship setaccount entityA relationship set is a mathematical relation among n 2 entities,each taken from entity sets(e1,e2,en)|e1 E1,e2 E2,en Enwhere(e1,e2,en)is a relationshipExample:(Hayes,A-102)depositor,Relationship Set borrower,Relationship Sets(Cont.),An attribute can also be property of a relationship set.For instance,the depositor relationship set between entity sets customer and account may have the attribute access-date,Degree of a Relationship Set,Refers to number of entity sets that participate in a relationship set.Relationship sets that involve two entity sets are binary(or degree two).Generally,most relationship sets in a database system are binary.Relationship sets may involve more than two entity sets.Relationships between more than two entity sets are rare.Most relationships are binary.(More on this later.),E.g.Suppose employees of a bank may have jobs(responsibilities)at multiple branches,with different jobs at different branches.Then there is a ternary relationship set between entity sets employee,job and branch,Mapping Cardinalities映射基数,Express the number of entities to which another entity can be associated via a relationship set.Most useful in describing binary relationship sets.For a binary relationship set the mapping cardinality must be one of the following types:One to oneOne to manyMany to oneMany to many,Mapping Cardinalities,One to one,One to many,Note:Some elements in A and B may not be mapped to any elements in the other set,Mapping Cardinalities,Many to one,Many to many,Note:Some elements in A and B may not be mapped to any elements in the other set,Mapping Cardinalities affect ER Design,Can make access-date an attribute of account,instead of a relationship attribute,if each account can have only one customer I.e.,the relationship from account to customer is many to one,or equivalently,customer to account is one to many,E-R Diagrams,Rectangles represent entity sets.Diamonds represent relationship sets.Lines link attributes to entity sets and entity sets to relationship sets.Ellipses represent attributesDouble ellipses represent multivalued attributes.Dashed ellipses denote derived attributes.Underline indicates primary key attributes(will study later),E-R Diagram With Composite,Multivalued,and Derived Attributes,Relationship Sets with Attributes,Roles,Entity sets of a relationship need not be distinctThe labels“manager”and“worker”are called roles;they specify how employee entities interact via the works-for relationship set.Roles are indicated in E-R diagrams by labeling the lines that connect diamonds to rectangles.Role labels are optional,and are used to clarify semantics of the relationship,Cardinality Constraints,We express cardinality constraints by drawing either a directed line(),signifying“one,”or an undirected line(),signifying“many,”between the relationship set and the entity set.E.g.:One-to-one relationship:A customer is associated with at most one loan via the relationship borrowerA loan is associated with at most one customer via borrower,One-To-Many Relationship,In the one-to-many relationship a loan is associated with at most one customer via borrower,a customer is associated with several(including 0)loans via borrower,Many-To-One Relationships,In a many-to-one relationship a loan is associated with several(including 0)customers via borrower,a customer is associated with at most one loan via borrower,Many-To-Many Relationship,A customer is associated with several(possibly 0)loans via borrowerA loan is associated with several(possibly 0)customers via borrower,Participation of an Entity Set in a Relationship Set,Total participation(indicated by double line):every entity in the entity set participates in at least one relationship in the relationship setE.g.participation of loan in borrower is total every loan must have a customer associated to it via borrowerPartial participation:some entities may not participate in any relationship in the relationship setE.g.participation of customer in borrower is partial,Alternative Notation for Cardinality Limits,Cardinality limits can also express participation constraints,Keys,A super key of an entity set is a set of one or more attributes whose values uniquely determine each entity.A candidate key of an entity set is a minimal super keyCustomer-id is candidate key of customeraccount-number is candidate key of accountAlthough several candidate keys may exist,one of the candidate keys is selected to be the primary key.,Keys for Relationship Sets,The combination of primary keys of the participating entity sets forms a super key of a relationship set.(customer-id,account-number)is the super key of depositorNOTE:this means a pair of entity sets can have at most one relationship in a particular relationship set.E.g.if we wish to track all access-dates to each account by each customer,we cannot assume a relationship for each access.We can use a multivalued attribute thoughMust consider the mapping cardinality of the relationship set when deciding the what are the candidate keys Need to consider semantics of relationship set in selecting the primary key in case of more than one candidate key,E-R Diagram with a Ternary Relationship,Cardinality Constraints on Ternary Relationship,We allow at most one arrow out of a ternary(or greater degree)relationship to indicate a cardinality constraintE.g.an arrow from works-on to job indicates each employee works on at most one job at any branch.If there is more than one arrow,there are two ways of defining the meaning.E.g a ternary relationship R between A,B and C with arrows to B and C could mean1.each A entity is associated with a unique entity from B and C or 2.each pair of entities from(A,B)is associated with a unique C entity,and each pair(A,C)is associated with a unique BEach alternative has been used in different formalismsTo avoid confusion we outlaw more than one arrow,Binary Vs.Non-Binary Relationships,Some relationships that appear to be non-binary may be better represented using binary relationshipsE.g.A ternary relationship parents,relating a child to his/her father and mother,is best replaced by two binary relationships,father and motherUsing two binary relationships allows partial information(e.g.only mother being know)But there are some relationships that are naturally non-binaryE.g.works-on,Converting Non-Binary Relationships to Binary Form,In general,any non-binary relationship can be represented using binary relationships by creating an artificial entity set.Replace R between entity sets A,B and C by an entity set E,and three relationship sets:1.RA,relating E and A 2.RB,relating E and B3.RC,relating E and CCreate a special identifying attribute for EAdd any attributes of R to E For each relationship(ai,bi,ci)in R,create 1.a new entity ei in the entity set E 2.add(ei,ai)to RA 3.add(ei,bi)to RB 4.add(ei,ci)to RC,Converting Non-Binary Relationships(Cont.),Also need to translate constraintsTranslating all constraints may not be possibleThere may be instances in the translated schema thatcannot correspond to any instance of RExercise:add constraints to the relationships RA,RB and RC to ensure that a newly created entity corresponds to exactly one entity in each of entity sets A,B and CWe can avoid creating an identifying attribute by making E a weak entity set(described shortly)identified by the three relationship sets,Design Issues,Use of entity sets vs.attributesChoice mainly depends on the structure of the enterprise being modeled,and on the semantics associated with the attribute in question.Use of entity sets vs.relationship setsPossible guideline is to designate a relationship set to describe an action that occurs between entitiesBinary versus n-ary relationship setsAlthough it is possible to replace any nonbinary(n-ary,for n 2)relationship set by a number of distinct binary relationship sets,a n-ary relationship set shows more clearly that several entities participate in a single relationship.Placement of relationship attributes,How about doing an ER design interactively on the board?Suggest an application to be modeled.,Weak Entity Sets,An entity set that does not have a primary key is referred to as a weak entity set.The existence of a weak entity set depends on the existence of a identifying entity set it must relate to the identifying entity set via a total,one-to-many relationship set from the identifying to the weak entity setIdentifying relationship depicted using a double diamondThe discriminator(or partial key)of a weak entity set is the set of attributes that distinguishes among all the entities of a weak entity set.The primary key of a weak entity set is formed by the primary key of the strong entity set on which the weak entity set is existence dependent,plus the weak entity sets discriminator.,Weak Entity Sets(Cont.),We depict a weak entity set by double rectangles.We underline the discriminator（分辨符）of a weak entity set with a dashed line.payment-number discriminator of the payment entity set Primary key for payment(loan-number,payment-number),Weak Entity Sets(Cont.),Note:the primary key of the strong entity set is not explicitly stored with the weak entity set,since it is implicit in the identifying relationship.If loan-number were explicitly stored,payment could be made a strong entity,but then the relationship between payment and loan would be duplicated by an implicit relationship defined by the attribute loan-number common to payment and loan,More Weak Entity Set Examples,In a university,a course is a strong entity and a course-offering can be modeled as a weak entityThe discriminator of course-offering would be semester(including year)and section-number(if there is more than one section)If we model course-offering as a strong entity we would model course-number as an attribute.Then the relationship with course would be implicit in the course-number attribute,Specialization,Top-down design process;we designate subgroupings within an entity set that are distinctive from other entities in the set.These subgroupings become lower-level entity sets that have attributes or participate in relationships that do not apply to the higher-level entity set.Depicted by a triangle component labeled ISA(E.g.customer“is a”person).Attribute inheritance a lower-level entity set inherits all the attributes and relationship participation of the higher-level entity set to which it is linked.,Specialization Example,Generalization,A bottom-up design process combine a number of entity sets that share the same features into a higher-level entity set.Specialization and generalization are simple inversions of each other;they are represented in an E-R diagram in the same way.The terms specialization and generalization are used interchangeably.,Specialization and Generalization(Contd.),Can have multiple specializations of an entity set based on different features.E.g.permanent-employee vs.temporary-employee,in addition to officer vs.secretary vs.tellerEach particular employee would be a member of one of permanent-employee or temporary-employee,and also a member of one of officer,secretary,or tellerThe ISA relationship also referred to as superclass-subclass relationship,Design Constraints on a Specialization/Generalization,Constraint on which entities can be members of a given lower-level entity set.condition-definedE.g.all customers over 65 years are members of senior-citizen entity set;senior-citizen ISA person.user-definedConstraint on whether or not entities may belong to more than one lower-level entity set within a single generalization.Disjointan entity can belong to only one lower-level entity setNoted in E-R diagram by writing disjoint next to the ISA triangleOverlappingan entity can belong to more than one lower-level entity set,Design Constraints on a Specialization/Generalization(Contd.),Completeness constraint-specifies whether or not an entity in the higher-level entity set must belong to at least one of the lower-level entity sets within a generalization.total:an entity must belong to one of the lower-level entity setspartial:an entity need not belong to one of the lower-level entity sets,Aggregation,Consider the ternary relationship works-on,which we saw earlier Suppose we want to record managers for tasks performed by an employee at a branch,Aggregation(Cont.),Relationship sets works-on and manages represent overlapping informationEvery manages relationship corresponds to a works-on relationshipHowever,some works-on relationships may not correspond to any manages relationships So we cant discard the works-on relationshipEliminate this redundancy via aggregationTreat relationship as an abstract entityAllows relationships between relationships Abstraction of relationship into new entityWithout introducing redundancy,the following diagram represents:An employee works on a particular job at a particular branch An employee,branch,job combination may have an associated manager,E-R Diagram With Aggregation,E-R Design Decisions,The use of an attribute or entity set to represent an object.Whether a real-world concept is best expressed by an entity set or a relationship set.The use of a ternary relationship versus a pair of binary relationships.The use of a strong or weak entity set.The use of specialization/generalization contributes to modularity in the design.The use of aggregation can treat the aggregate entity set as a single unit without concern for the details of its internal structure.,E-R Diagram for a Banking Enterprise,How about doing another ER design interactively on the board?,Summary of Symbols Used in E-R Notation,Summary of Symbols(