Chapter 6: Entity-Relationship Model

Chapter 6: Entity-Relationship Model

اسلاید 1: Chapter 6: Entity-Relationship Model

اسلاید 2: Chapter 6: Entity-Relationship ModelDesign ProcessModelingConstraintsE-R Diagram Design Issues Weak Entity Sets Extended E-R FeaturesDesign of the Bank DatabaseReduction to Relation SchemasDatabase DesignUML

اسلاید 3: ModelingA 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

اسلاید 4: Entity Sets customer and loancustomer_id customer_ customer_ customer_ loan_ amount name street city number

اسلاید 5: 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 6: Relationship Set borrower

اسلاید 7: 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 8: 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 9: 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 10: Composite Attributes

اسلاید 11: 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 12: Mapping CardinalitiesOne to oneOne to manyNote: Some elements in A and B may not be mapped to any elements in the other set

اسلاید 13: 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 14: 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 15: 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 16: 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 17: E-R Diagram With Composite, Multivalued, and Derived Attributes

اسلاید 18: Relationship Sets with Attributes

اسلاید 19: 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 20: 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 21: 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 22: 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 23: 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 24: 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 25: Alternative Notation for Cardinality LimitsCardinality limits can also express participation constraints

اسلاید 26: E-R Diagram with a Ternary Relationship

اسلاید 27: 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 28: 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 29: Binary Vs. Non-Binary RelationshipsSome 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: works_on

اسلاید 30: Converting Non-Binary Relationships to Binary FormIn 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

اسلاید 31: Converting Non-Binary Relationships (Cont.)Also need to translate constraintsTranslating all constraints may not be possibleThere may be instances in the translated schema that cannot 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

اسلاید 32: Mapping Cardinalities affect ER DesignCan make access-date an attribute of account, instead of a relationship attribute, if each account can have only one customer That is, the relationship from account to customer is many to one, or equivalently, customer to account is one to many

اسلاید 33: How about doing an ER design interactively on the board? Suggest an application to be modeled.

اسلاید 34: Weak Entity SetsAn 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 set’s discriminator.

اسلاید 35: Weak Entity Sets (Cont.)We depict a weak entity set by double rectangles.We underline the discriminator 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)

اسلاید 36: 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

اسلاید 37: More Weak Entity Set ExamplesIn 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

اسلاید 38: Extended E-R Features: SpecializationTop-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.

اسلاید 39: Specialization Example

اسلاید 40: Extended ER Features: GeneralizationA 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.

اسلاید 41: Specialization and Generalization (Cont.)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

اسلاید 42: Design Constraints on a Specialization/GeneralizationConstraint on which entities can be members of a given lower-level entity set.condition-definedExample: 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

اسلاید 43: Design Constraints on a Specialization/Generalization (Cont.)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

اسلاید 44: 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

اسلاید 45: 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 can’t 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

اسلاید 46: E-R Diagram With Aggregation

اسلاید 47: E-R Design DecisionsThe 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.

اسلاید 48: E-R Diagram for a Banking Enterprise

اسلاید 49: How about doing another ER design interactively on the board?

اسلاید 50: Summary of Symbols Used in E-R Notation

اسلاید 51: Summary of Symbols (Cont.)

اسلاید 52: Reduction to Relation SchemasPrimary keys allow entity sets and relationship sets to 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.

اسلاید 53: Representing Entity Sets as SchemasA strong entity set reduces to a schema with the same attributes.A weak entity set becomes a table that includes a column for the primary key of the identifying strong entity setpayment = ( loan_number, payment_number, payment_date, payment_amount )

اسلاید 54: Representing Relationship Sets as SchemasA 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 borrowerborrower = (customer_id, loan_number )

اسلاید 55: Redundancy of SchemasMany-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 account_branch, add an attribute branch_name to the schema arising from entity set account

اسلاید 56: 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 payment schema already contains the attributes that would appear in the loan_payment schema (i.e., loan_number and payment_number).

اسلاید 57: Composite and Multivalued AttributesComposite attributes are flattened out by creating a separate attribute for each component attributeExample: given entity set customer 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_nameA 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 dependent_names of employee is represented by a schema: employee_dependent_names = ( employee_id, dname) Each value of the multivalued attribute maps to a separate tuple of the relation on schema EMFor example, an employee entity with primary key 123-45-6789 and dependents Jack and Jane maps to two tuples: (123-45-6789 , Jack) and (123-45-6789 , Jane)

اسلاید 58: Representing Specialization via SchemasMethod 1: Form a schema for the higher-level entity Form a schema for each lower-level entity set, include primary key of higher-level entity set and local attributes schema attributes person name, street, city customer name, credit_rating employee name, salaryDrawback: getting information about, an employee requires accessing two relations, the one corresponding to the low-level schema and the one corresponding to the high-level schema

اسلاید 59: Representing Specialization as Schemas (Cont.)Method 2: Form a schema for each entity set with all local and inherited attributes schema attributes personname, street, city customername, street, city, credit_rating employee name, street, city, salary If specialization is total, the schema for the generalized entity set (person) not required to store informationCan be defined as a “view” relation containing union of specialization relationsBut explicit schema may still be needed for foreign key constraintsDrawback: street and city may be stored redundantly for people who are both customers and employees

اسلاید 60: Schemas Corresponding to AggregationTo represent aggregation, create a schema containingprimary key of the aggregated relationship,the primary key of the associated entity setany descriptive attributes

اسلاید 61: Schemas Corresponding to Aggregation (Cont.)For example, to represent aggregation manages between relationship works_on and entity set manager, create a schema manages (employee_id, branch_name, title, manager_name)Schema works_on is redundant provided we are willing to store null values for attribute manager_name in relation on schema manages

اسلاید 62: UMLUML: Unified Modeling LanguageUML has many components to graphically model different aspects of an entire software systemUML Class Diagrams correspond to E-R Diagram, but several differences.

اسلاید 63: Summary of UML Class Diagram Notation

اسلاید 64: UML Class Diagrams (Cont.)Entity sets are shown as boxes, and attributes are shown within the box, rather than as separate ellipses in E-R diagrams.Binary relationship sets are represented in UML by just drawing a line connecting the entity sets. The relationship set name is written adjacent to the line. The role played by an entity set in a relationship set may also be specified by writing the role name on the line, adjacent to the entity set. The relationship set name may alternatively be written in a box, along with attributes of the relationship set, and the box is connected, using a dotted line, to the line depicting the relationship set. Non-binary relationships drawn using diamonds, just as in ER diagrams

اسلاید 65: UML Class Diagram Notation (Cont.)*Note reversal of position in cardinality constraint depiction*Generalization can use merged or separate arrows independent of disjoint/overlappingoverlappingdisjoint

اسلاید 66: UML Class Diagrams (Contd.)Cardinality constraints are specified in the form l..h, where l denotes the minimum and h the maximum number of relationships an entity can participate in.Beware: the positioning of the constraints is exactly the reverse of the positioning of constraints in E-R diagrams.The constraint 0..* on the E2 side and 0..1 on the E1 side means that each E2 entity can participate in at most one relationship, whereas each E1 entity can participate in many relationships; in other words, the relationship is many to one from E2 to E1.Single values, such as 1 or * may be written on edges; The single value 1 on an edge is treated as equivalent to 1..1, while * is equivalent to 0..*.

اسلاید 67: End of Chapter 2

اسلاید 68: E-R Diagram for Exercise 2.10

اسلاید 69: E-R Diagram for Exercise 2.15

اسلاید 70: E-R Diagram for Exercise 2.22

اسلاید 71: E-R Diagram for Exercise 2.15

اسلاید 72: Existence DependenciesIf the existence of entity x depends on the existence of entity y, then x is said to be existence dependent on y.y is a dominant entity (in example below, loan)x is a subordinate entity (in example below, payment)loan-paymentpaymentloanIf a loan entity is deleted, then all its associated payment entities must be deleted also.

اسلاید 73: Figure 6.8

اسلاید 74: Figure 6.15

اسلاید 75: Figure 6.16

اسلاید 76: Figure 6.26

اسلاید 77: Figure 6.27

اسلاید 78: Figure 6.28

اسلاید 79: Figure 6.29

اسلاید 80: Figure 6.30

اسلاید 81: Figure 6.31

اسلاید 82: Alternative E-R Notations Figure 6.24