پاورپوینت کامل Chapter 6: Entity-Relationship Model 89 اسلاید در PowerPoint

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پاورپوینت کامل Chapter 6: Entity-Relationship Model 89 اسلاید در PowerPoint

اسلاید ۴: Entity Sets customer and loancustomer_id customer_ customer_ customer_ loan_ amount name street city number

اسلاید ۵: Relationship SetsA relationship is an association among several entitiesExample: HayesdepositorA-102 customer 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 En} where (e1, e2, …, en) is a relationshipExample: (Hayes, A-102) depositor

اسلاید ۶: Relationship Set borrower

اسلاید ۷: 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

اسلاید ۸: Degree of a Relationship SetRefers 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.)Example: 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

اسلاید ۹: AttributesAn 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 attributesExample: multivalued attribute: phone_numbersDerived attributesCan be computed from other attributesExample: age, given date_of_birthExample: customer = (customer_id, customer_name, customer_street, customer_city ) loan = (loan_number, amount )

اسلاید ۱۰: Composite Attributes

اسلاید ۱۱: Mapping Cardinality ConstraintsExpress 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

اسلاید ۱۲: Mapping CardinalitiesOne to oneOne to manyNote: Some elements in A and B may not be mapped to any elements in the other set

اسلاید ۱۳: Mapping Cardinalities Many to oneMany to manyNote: Some elements in A and B may not be mapped to any elements in the other set

اسلاید ۱۴: KeysA 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.

اسلاید ۱۵: Keys for Relationship SetsThe 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. Example: 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

اسلاید ۱۶: E-R DiagramsRectangles 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)

اسلاید ۱۷: E-R Diagram With Composite, Multivalued, and Derived Attributes

اسلاید ۱۸: Relationship Sets with Attributes

اسلاید ۱۹: RolesEntity 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

اسلاید ۲۰: Cardinality ConstraintsWe 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 customer is associated with at most one loan via the relationship borrowerA loan is associated with at most one customer via borrower

اسلاید ۲۱: One-To-Many RelationshipIn 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

اسلاید ۲۲: Many-To-One RelationshipsIn 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

اسلاید ۲۳: Many-To-Many RelationshipA customer is associated with several (possibly 0) loans via borrowerA loan is associated with several (possibly 0) customers via borrower

اسلاید ۲۴: Participation of an Entity Set in a Relationship SetTotal 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 setExample: participation of customer in borrower is partial

اسلاید ۲۵: Alternative Notation for Cardinality LimitsCardinality limits can also express participation constraints

اسلاید ۲۶: E-R Diagram with a Ternary Relationship

اسلاید ۲۷: Cardinality Constraints on Ternary RelationshipWe 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 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

اسلاید ۲۸: Design IssuesUse of entity sets vs. attributes Choice 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 sets Possible guideline is to designate a relationship set to describe an action that occurs between entitiesBinary versus n-ary relationship sets Although 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

اسلاید ۲۹: Binary Vs. Non-Binary RelationshipsSome relationships that appear to be non-binary may be better represented using binary relationshipsE.g. A te