©silberschatz, korth and sudarshan6.1database system concepts chapter 6: entity-relationship model...

©Silberschatz, Korth and Sudarshan6.1Database System Concepts

Chapter 6: Entity-Relationship ModelChapter 6: Entity-Relationship Model

Entity Sets Attributes Relationship Sets Mapping Cardinality Keys Extended E-R Features Design of an E-R Database Schema Reduction of an E-R Schema to Tables


Entity SetsEntity Sets

A database can be modeled as a collection of: entities, and relationships among those entities.

An entity is an object that is distinguishable from other objects. Specific person, specific company, specific automobile

Entities have attributes People have names and addresses An entity and its’ attributes are represented by a tuple

(342-97-4873, Smith, Main, Orlando)

An entity set is a set of entities of the same type that share the same properties and attributes. Set of all persons, set of all companies, set of all

automobiles


Entity Sets Entity Sets customercustomer and and loanloan

customer-id customercustomer- customer- loan- amount name street city number


AttributesAttributes

The set of permitted values for an attribute is call the domain of that attribute.

Attributes can be one of several types: Simple

• height (in inches), weight (in ounces), last-name

Composite• name, address

Single-valued• any of the above

Multi-valued attributes:• phone-numbers, dependents, hobbies

Derived attributes• Can be computed from other attributes• Age is derived from date-of-birth


Attributes, Cont.Attributes, Cont.

Note that during modeling we are focused on what the relevant data is, and not whether or how it will be in the database. Age vs. date-of-birth


Composite AttributesComposite Attributes


Relationship SetsRelationship Sets

A relationship is an association among two (or more) entities Hayes is a depositor for account A-102 The relationship is denoted by the tuple (Hayes, A-

102)

A 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 relationship depositor is a relationship set, e.g., (Hayes, A-

102) depositor


Relationship Set Relationship Set borrowerborrower

Relationship sets can be visualized graphically.


Relationship Sets (Cont.)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


Relationship SetsRelationship Setswith Attributes, Cont.with Attributes, Cont.

Another example of a relationship set having attributes: Entities: Student and Course Relationship: Has-Taken

Where does the attribute grade go?


Degree of a Relationship SetDegree of a Relationship Set

The number of entity sets that participate in a relationship set is referred to as the degree of that relationship set.

Relationship sets that involve two entity sets are called binary. Most relationship sets in a database system are binary.

Example of a ternary relationship set. Suppose employees of a bank may have jobs at multiple

branches, with different jobs at different branches. In such a case there is a ternary relationship set

between employee, job and branch.


Mapping CardinalitiesMapping Cardinalities

The mapping cardinality of a relationship set expresses the number of entities to which one entity can be associated via the relationship set.

Most useful in describing binary relationship sets.

For a binary relationship there are four types: One to one – legal US marriages One to many – academic advisors (at most one major) Many to one – same as one-to-many Many to many – depositors


Mapping CardinalitiesMapping 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 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 DesignMapping Cardinalities affect ER Design

access-date could be an attribute of account instead of a relationship attribute if each account can have only one customer, i.e., if the relationship is one-to-many.


E-R DiagramsE-R Diagrams

Rectangles - entity sets Diamonds - relationship sets Ellipses - attributes:

Double ellipses - multi-valued Dashed ellipses – derived

Underlined - primary key attributes (will discuss later) Lines - connect attributes to entity sets, and

entity sets to relationship sets.


E-R Diagram with Composite, Multivalued,E-R Diagram with Composite, Multivalued,and Derived Attributesand Derived Attributes


Relationship Sets with AttributesRelationship Sets with Attributes


RolesRoles

The 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 a relationship set Particularly helpful when entity sets of a relationship are not

distinct


Cardinality ConstraintsCardinality 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.

If borrower were a one-to-one relationship: A customer would be associated with at most one loan A loan would be associated with at most one customer


One-To-Many RelationshipOne-To-Many Relationship

If borrower were a one-to-many relationship then a loan would be associated with at most one customer, and a customer would be associated with zero or more loans.


Many-To-One RelationshipsMany-To-One Relationships

If borrower were a many-to-one relationship then a loan would be associated with zero or more customers, and a customer would be associated with at most one loan.


Many-To-Many RelationshipMany-To-Many Relationship

A customer is associated with zero or more loans via borrower

A loan is associated with zero or more customers via borrower


Participation of an Entity SetParticipation of an Entity Setin a Relationship Setin a Relationship Set

If every entity in an entity set participates in a relationship set, then that entity set is said to have total participation in the relationship set. Total participation is indicated by a double-line Participation of loan in borrower is total since

every loan must have at least one customer associated with it via borrower

If participation in a relationship set is optional for some entities then that entity set is said to have partial participation in the relationship set. Participation of customer in borrower is partial


Another ExampleAnother Exampleof Total Participationof Total Participation

Consider a relationship between entity sets Student and Faculty called Advises.

Assuming each student has exactly one major: Every student has exactly one faculty advisor Some faculty don’t advise students

In this case the Advises relationship set is: One to many from Faculty to Student Student participation is total

In most universities, the Advises relationship is more likely many-to-many, with the Student participation being total.


Alternative NotationAlternative Notationfor Cardinality Limitsfor Cardinality Limits

Specific cardinality limits can also express participation constraints


KeysKeys

A super key of an entity set is a set of one or more attributes whose values uniquely identify each entity.

A candidate key of an entity set is a minimal super key customer-id is candidate key of customer account-number is candidate key of account

Although several candidate keys may exist, one of the candidate keys is selected to be the primary key. All others are referred to as secondary keys

Later we will also discuss foreign keys and search keys.


Other Examples of KeysOther Examples of Keys

Student = (SS#, Name, Date-of-Birth, Status)SS# - Super key and candidate keySS#, Name, DOB - Super key, but not candidate keySS#, Status - Super key, but not candidate keyName - Neither

Payment = (Payment#, Loan#, Date-Made, ID#)ID# - Super key, and candidate keyPayment#, Loan# - Super key, and candidate keyID#, Date-Made - Super key, but not candidate keyDate - Neither


Keys in an ER DiagramKeys in an ER Diagram

A primary key of an entity set is specified in an ER diagram by underlining the key attributes.


Keys for Relationship SetsKeys 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

depositor• Notes:

– This reflects the fact that a pair of entity sets can have at most one relationship in a particular relationship set.

– If we wish to track all access-dates to each account by each customer, we cannot assume a relationship for each access. For this we could use a multi-valued attribute, however.

Question: why primary and super in the above?

The mapping cardinality of a relationship set will determine, in part, what the candidate keys of the relationship set are.


Design IssuesDesign Issues

Use of entity sets vs. attributes Is Telephone-Number an attribute or an entity set? Choice mainly depends on the structure of the enterprise being

modeled, and on the semantics/meaning associated with the attribute in question.

Use of entity sets vs. relationship sets Is Loan an entity set or a relationship set? Possible guideline is to designate a relationship set to

describe an action that occurs between entities

Binary versus n-ary relationship sets E.g., Family, as opposed to mother, father, brother, sister 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

How about doing an ER design How about doing an ER design interactively on the board?interactively on the board?

Suggest an application to be modeled.Suggest an application to be modeled.


Weak Entity SetsWeak Entity Sets

An entity set that has a primary key is referred to as a strong entity set.

An entity set that does not have a primary key is referred to as a weak entity set.

A weak entity set is typically associated with an identifying entity set (which is usually strong) via a total, one-to-many relationship.


Weak Entity Sets, Cont.Weak Entity Sets, Cont.

The discriminator (or partial key) of a weak entity set is the set of attributes that distinguishes among all entities of the weak entity set associated with one identifying entity.

The primary key of a weak entity set has two parts: The primary key of the associated identifying

entity set The weak entity set’s discriminator


Weak Entity Sets (Cont.)Weak Entity Sets (Cont.)

A weak entity set is represented by double rectangles. The discriminator is underlined with a dashed line. Primary key for payment – (loan-number, payment-number)


Weak Entity Sets (Cont.)Weak Entity Sets (Cont.)

Note: the primary key of the strong entity set is not explicitly specified in the weak entity set, since it is implicit in the identifying relationship.

If loan-number were explicitly specified: Payment would be a strong entity set The nature of the relationship between payment and

loan would not be as clear


More Weak Entity Set ExamplesMore Weak Entity Set Examples

In a university, a course is a strong entity and a course-offering can be modeled as a weak entity.

Course = (course-number, name, description)

Course-offering = (semester, section-number, instructor)

The discriminator of course-offering would be semester (including year) and section-number (if there is more than one section).

If course-offering were modeled as a strong entity then it would have course-number as an attribute. The relationship with course would then be implicit in the

course-number attribute.


Specialization andSpecialization andGeneralization ExampleGeneralization Example


SpecializationSpecialization

Top-down design process; we designate sub-groupings within an entity set that are distinctive from other entities in the set. These sub-groupings are referred to as lower-level entity sets.

The lower-level entity sets may: Have attributes that do not apply to the higher-level entity set. Participate in relationships that do not apply to the higher-level

entity set.

A lower-level entity set is said to inherit all the attributes and relationships from the higher-level entity set to which it is linked.

Depicted by a triangle component labeled ISA (E.g. customer “is a” person).


GeneralizationGeneralization

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.

The ISA relationship also referred to as a superclass-subclass relationship.


SpecializationSpecializationand Generalization, Cont.and Generalization, Cont.

Multiple specializations of an entity set based on different features are possible. permanent-employee vs. temporary-employee In addition to officer vs. secretary vs. teller

Each particular employee would be a member of: One of permanent-employee or temporary-employee,

and One of officer, secretary, or teller


Design Constraints on Design Constraints on Specialization/GeneralizationSpecialization/Generalization

In a specialization/generalization relationship: If an entity can belong to at most one lower-level

entity set then that relationship is said to be disjoint.

If an entity can belong to more than one lower-level entity set then that relationship is said to be overlapping.

If an entity must belong to one of the lower-level entity sets then that relationship is said to be total.

If an entity need not belong to one of the lower-level entity sets then that relationship is said to be partial.

The above could be noted in any number of ways on the diagram, e.g., by writing “disjoint” next to the relationship.


Design IssuesDesign Issues

Now, there are even more design options: The use of a strong or weak entity set. The use of specialization/generalization.


E-R DiagramE-R Diagramfor a Banking Enterprisefor a Banking Enterprise


Summary of SymbolsSummary of SymbolsUsed in E-R NotationUsed in E-R Notation


Summary of Symbols (Cont.)Summary of Symbols (Cont.)


Alternative E-R NotationsAlternative E-R Notations


UMLUML

UML: Unified Modeling Language

UML has many components to graphically model different aspects of an entire software system

UML Class Diagrams correspond to E-R Diagram, but several differences.


Summary of UML Class Diagram NotationSummary of UML Class Diagram Notation


UML Class Diagrams (Contd.)UML Class Diagrams (Contd.)

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


UML Class Diagram Notation (Cont.)UML Class Diagram Notation (Cont.)

*Note reversal of position in cardinality constraint depiction*Generalization can use merged or separate arrows independent of disjoint/overlapping

overlapping

disjoint


UML Class Diagrams (Contd.)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..*.


Reduction ofReduction ofan E-R Schema to Tablesan E-R Schema to Tables

An E-R diagram can be converted to a collection of tables, i.e., a relational database. Each entity set is converted to its’ own

table. Each relationship set can be (but may or may

not be) converted to its’ own table.

Each table has a number of columns, which generally corresponding to the attributes in the corresponding entity or relationship set.


Representing Entity Sets as TablesRepresenting Entity Sets as Tables

A strong entity set reduces to a table with the same attributes.


Composite andComposite andMulti-valued AttributesMulti-valued Attributes

Composite attributes are broken up: Example: attribute name with components first-name and last-

name becomes two attributes in the corresponding table: name.first-name and name.last-name.

A multi-valued attribute M of entity E is represented by a new table with the following attributes: The primary key of E An attribute corresponding to multi-valued attribute M

Example:Entity Set:employee with attributes id#, name, phone#, dependentsTables:employee (id#, name, phone#)dependent (id#, dname)


Representing Weak Entity SetsRepresenting Weak Entity Sets

A weak entity set becomes a table that includes a column for the primary key of the identifying strong entity set.


Representing RelationshipRepresenting RelationshipSets as TablesSets as Tables

A many-to-many relationship set is represented as a table with columns for the primary keys of the two participating entity sets, and any descriptive attributes of the relationship set.

Example: for the borrower relationship set:


Representing RelationshipRepresenting RelationshipSets as Tables, cont.Sets as Tables, cont.

A Many-to-one relationship set can be represented just like a many-to-many relationship.

Technically this is not necessary, and in some cases it does not result in a good design.

Example:Entity Sets:

account with attributes account-number, balance

branch with attributes branch-name, branch-city, assets

Relationship Set (total, many-to-one from account to branch):

account-branch with attributes account-number,

branch-name



The preceding could be converted to tables directly, or as follows:

account (account-number, balance, branch-name)

branch (branch-name, branch-city, assets)

Since the above relationship is total, a distinct table for the relationship set is probably not necessary.



For one-to-one relationship sets, the extra attribute can be added to either of the tables corresponding to the two entity sets.

Similarly, other relationship attributes would be treated similarly.

Note that either of the above could introduce null values if the relationship is not total.


Representing SpecializationRepresenting Specializationas Tablesas Tables

Note: This discussion assumes a 2-level inheritance hierarchy. Exercise: Generalize it to an arbitrarily deep

hierarchy.

Method 1: Form a table for the higher level entity set. Form a table for each lower level entity set, including

the primary key of the higher level entity set and local attributes.

table attributesperson name, street, citycustomer name, credit-ratingemployee name, salary

One Drawback: getting information about specific entities requires accessing two tables


Representing SpecializationRepresenting Specializationas Tables cont.as Tables cont.

Method 2: Form a table for each entity set with all local and inherited

attributestable table attributes

personname, street, citycustomername, street, city, credit-ratingemployee name, street, city, salary

This method can be redundant: Some duplication for persons who are both customers and

employees. If specialization is total, the table for the generalized

entity is redundant.• Explicit table may still be needed for foreign key constraints.

• For convenience, the generalized entity can an be defined as a “view” relation containing union of specialization tables.


Representing SpecializationRepresenting Specializationas Tablesas Tables

Method 3: (not presented in the book) Form one table for the higher level entity set This table has one column for every attribute in every

subclass.table attributesperson name, street, city, credit-rating,

salary

Optionally, include a type attribute that indicates which subclass the stored entity belongs to.

One Drawback: Contains multiple nullable attributes.

Sometimes referred to as a “junk drawer”


Process IssuesProcess Issues

The book also defines the following (software engineering steps): Specification of user requirements Conceptual design Specification of functional requirements Logical design Physical design

These development/design steps, i.e., the process, is typically company specific.

End of Chapter 6End of Chapter 6


E-RE-R Diagram with a Ternary Relationship Diagram with a Ternary Relationship


Cardinality ConstraintsCardinality Constraintson Ternary Relationship Setson Ternary Relationship Sets

We allow at most one arrow out of a ternary (or greater degree) relationship to indicate a cardinality constraint 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 B

Each alternative has been used in different formalisms To avoid confusion we outlaw more than one arrow


Binary Vs. Non-Binary RelationshipsBinary Vs. Non-Binary Relationships Some relationships that appear to be non-binary

may be better represented using binary relationships E.g. A ternary relationship parents, relating a

child to his/her father and mother, is best replaced by two binary relationships, father and mother• Using two binary relationships allows partial information (e.g. only mother being know)

But there are some relationships that are naturally non-binary• E.g. works-on


Converting Non-Binary Relationships to Binary Converting Non-Binary Relationships to Binary FormForm

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 C Create a special identifying attribute for E Add 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 Converting Non-Binary Relationships, Cont.Relationships, Cont.

Also need to translate constraints Translating all constraints may not be possible There may be instances in the translated schema that

cannot correspond to any instance of R• Exercise: 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 C

We can avoid creating an identifying attribute by making E a weak entity set (described shortly) identified by the three relationship sets


E-R Diagram for Exercise 2.10E-R Diagram for Exercise 2.10


Existence DependenciesExistence Dependencies

If 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-payment paymentloan

If a loan entity is deleted, then all its associated payment entities must be deleted also.


AggregationAggregation 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.)Aggregation (Cont.) Relationship sets works-on and manages represent

overlapping information Every manages relationship corresponds to a works-on

relationship However, some works-on relationships may not correspond

to any manages relationships • So we can’t discard the works-on relationship

Eliminate this redundancy via aggregation Treat relationship as an abstract entity Allows relationships between relationships Abstraction of relationship into new entity


Aggregation (Cont.)Aggregation (Cont.) Without introducing redundancy, the following

diagram represents: An employee works on a particular job at a particular

branch An employee, branch, job combination may (or may not)

have an associated manager


E-R Diagram With AggregationE-R Diagram With Aggregation


Relations Corresponding to Relations Corresponding to AggregationAggregation

To represent aggregation, create a table containing The primary key of the aggregated relationship,

The primary key of the associated entity set

Any descriptive attributes


Relations Corresponding to Relations Corresponding to Aggregation (Cont.)Aggregation (Cont.)

Example: to represent aggregation manages between relationship works-on and entity set manager, create a table: manages(employee-id, branch-name, title, manager-name) Note: manager-name may be null. If this is acceptable, then table works-on is not required.

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