《数据库系统概念教学课件》ch.ppt
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1、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 en
2、tities,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
3、.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 instructo
4、r 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 i
5、nstance,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 datab
6、ase 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,At
7、tributes,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
8、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 asso
9、ciated 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
10、 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
11、 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
12、 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
13、 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,S
14、uppose 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 attribut
15、e 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 entit
16、y 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
17、”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 t
18、he 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 adv
19、isor,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 inst
20、ructor 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(po
21、ssibly 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
22、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
23、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
24、 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
25、)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 referr
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