《数据库系统概念教学课件》ch.ppt

Chapter 7:Entity-Relationship Model,Chapter 7:Entity-Relationship Model,Design ProcessModelingConstraintsE-R Diagram Design Issues Weak Entity Sets Extended E-R FeaturesDesign of the Bank DatabaseReduction to Relation SchemasDatabase DesignUML,Modeling,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 instructor and student,instructor_ID instructor_name student-ID student_name,Relationship Sets,A relationship is an association among several entitiesExample:44553(Peltier)advisor 22222(Einstein)student entityrelationship set instructor 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:(44553,22222)advisor,Relationship Set advisor,Relationship Sets(Cont.),An attribute can also be property of a relationship set.For instance,the advisor relationship set between entity sets instructor and student may have the attribute date which tracks when the student started being associated with the advisor,Degree of a Relationship Set,binary relationshipinvolve two entity sets(or degree two).most relationship sets in a database system are binary.Relationships between more than two entity sets are rare.Most relationships are binary.(More on this later.)Example:students work on research projects under the guidance of an instructor.relationship proj_guide is a ternary relationship between instructor,student,and project,Attributes,An entity is represented by a set of attributes,that is descriptive properties possessed by all members of an entity set.Example:instructor=(ID,name,street,city,salary)course=(course_id,title,credits)Domain the set of permitted values for each attribute Attribute types:Simple and composite attributes.Single-valued and multivalued attributesExample:multivalued attribute:phone_numbersDerived attributesCan be computed from other attributesExample:age,given date_of_birth,Composite Attributes,Mapping Cardinality Constraints,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,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 keyID is candidate key of instructorcourse_id is candidate key of courseAlthough 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.(s_id,i_id)is the super key of advisorNOTE:this means a pair of entity sets can have at most one relationship in a particular relationship set.Example:if we wish to track multiple meeting dates between a student and her advisor,we cannot assume a relationship for each meeting.We can use a multivalued attribute thoughMust consider the mapping cardinality of the relationship set when deciding 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,Redundant Attributes,Suppose we have entity setsinstructor,with attributes including dept_namedepartmentand a relationshipinst_dept relating instructor and departmentAttribute dept_name in entity instructor is redundant since there is an explicit relationship inst_dept which relates instructors to departmentsThe attribute replicates information present in the relationship,and should be removed from instructorBUT:when converting back to tables,in some cases the attribute gets reintroduced,as we will see.,E-R Diagrams,Rectangles represent entity sets.Diamonds represent relationship sets.Attributes listed inside entity rectangleUnderline indicates primary key attributes,Entity With Composite,Multivalued,and Derived Attributes,Relationship Sets with Attributes,Roles,Entity sets of a relationship need not be distinctEach occurrence of an entity set plays a“role”in the relationshipThe labels“course_id”and“prereq_id”are called roles.,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.One-to-one relationship:A student is associated with at most one instructor via the relationship advisorA student is associated with at most one department via stud_dept,One-to-One Relationship,one-to-one relationship between an instructor and a studentan instructor is associated with at most one student via advisor and a student is associated with at most one instructor via advisor,One-to-Many Relationship,one-to-many relationship between an instructor and a student an instructor is associated with several(including 0)students via advisor a student is associated with at most one instructor via advisor,Many-to-One Relationships,In a many-to-one relationship between an instructor and a student,an instructor is associated with at most one student via advisor,and a student is associated with several(including 0)instructors via advisor,Many-to-Many Relationship,An instructor is associated with several(possibly 0)students via advisorA student is associated with several(possibly 0)instructors via advisor,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 section in sec_course is total every section must have an associated coursePartial participation:some entities may not participate in any relationship in the relationship setExample:participation of instructor in advisor is partial,Alternative Notation for Cardinality Limits,Cardinality limits can also express participation constraints,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 proj_guide to instructor indicates each student has at most one guide for a projectIf 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 mean 1.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,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 setIt 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 underline the discriminator of a weak entity set with a dashed line.We put the identifying relationship of a weak entity in a double diamond.Primary key for section(course_id,sec_id,semester,year),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 course_id were explicitly stored,section could be made a strong entity,but then the relationship between section and course would be duplicated by an implicit relationship defined by the attribute course_id common to course and section,E-R Diagram for a University Enterprise,Reduction to Relational Schemas,Reduction to Relation Schemas,Entity sets and relationship sets can be expressed uniformly as relation schemas that represent the contents of the database.A database which conforms to an E-R diagram can be represented by a collection of schemas.For each entity set and relationship set there is a unique schema that is assigned the name of the corresponding entity set or relationship set.Each schema has a number of columns(generally corresponding to attributes),which have unique names.,Representing Entity Sets With Simple Attributes,A strong entity set reduces to a schema with the same attributesstudent(ID,name,tot_cred)A weak entity set becomes a table that includes a column for the primary key of the identifying strong entity set section(course_id,sec_id,sem,year),Representing Relationship Sets,A many-to-many relationship set is represented as a schema with attributes for the primary keys of the two participating entity sets,and any descriptive attributes of the relationship set.Example:schema for relationship set advisoradvisor=(s_id,i_id),Redundancy of Schemas,Many-to-one and one-to-many relationship sets that are total on the many-side can be represented by adding an extra attribute to the“many”side,containing the primary key of the“one”sideExample:Instead of creating a schema for relationship set inst_dept,add an attribute dept_name to the schema arising from entity set instructor,Redundancy of Schemas(Cont.),For one-to-one relationship sets,either side can be chosen to act as the“many”sideThat is,extra attribute can be added to either of the tables corresponding to the two entity sets If participation is partial on the“many”side,replacing a schema by an extra attribute in the schema corresponding to the“many”side could result in null valuesThe schema corresponding to a relationship set linking a weak entity set to its identifying strong entity set is redundant.Example:The section schema already contains the attributes that would appear in the sec_course schema,Composite and Multivalued Attributes,Composite attributes are flattened out by creating a separate attribute for each component attributeExample:given entity set instructor with composite attribute name with component attributes first_name and last_name the schema corresponding to the entity set has two attributes name_first_name and name_last_namePrefix omitted if there is no ambiguityIgnoring multivalued attributes,extended instructor schema isinstructor(ID,first_name,middle_initial,last_name,street_number,street_name,apt_number,city,state,zip_code,date_of_birth),Composite and Multivalued Attributes,A multivalued attribute M of an entity E is represented by a separate schema EMSchema EM has attributes corresponding to the primary key of E and an attribute corresponding to multivalued attribute MExample:Multivalued attribute phone_number of instructor is represented by a schema:inst_phone=(ID,phone_number)Each value of the multivalued attribute maps to a separate tuple of the relation on schema EMFor example,an instructor entity with primary key 22222 and phone numbers 456-7890 and 123-4567 maps to two tuples:(22222,456-7890)and(22222,123-4567),Multivalued Attributes(Cont.),Special case:entity time_slot has only one attribute other than the primary-key attribute,and that attribute is multivaluedOptimization:Dont create the relation corresponding to the entity,just create the one corresponding to the multivalued attributetime_slot(time_slot_id,day,start_time,end_time)Caveat:time_slot attribute of section(from sec_time_slot)cannot be a foreign key due to this optimization,Design Issues,Use of entity sets vs.attributesUse of phone as an entity allows extra information about phone numbers(plus multiple phone numbers),Design Issues,Use of entity sets vs.relationship setsPossible guideline is to designate a relationship set to describe an action that occurs between entities,Design Issues,Binary 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 e.g.,attribute date as attribute of advisor or as attribute of student,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-binaryExample:proj_guide,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 B 3.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,Extended ER Features,Extended E-R Features: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