D ATABASE S YSTEMS I W EEK 2 T HE E NTITY -R ELATIONSHIP M ODEL.

DATABASE SYSTEMS I

WEEK 2THE ENTITY-RELATIONSHIP MODEL

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SYLLABUS Class Time and Location:

Tue 14:30-16:20 AQ3005 Thu 14:30-15:20 AQ3003

Course Website: http://www.cs.sfu.ca/CC/354/rfrank/

Instructor: Richard Frank, PhD Email: [email protected] Office Hours

Location: TASC 9205 Time: Tuesday, 1:30pm-2:30pm

TA: Ankit Gupta Email: [email protected] Office Hours

Location: ASB9838_TA_1 Time: Monday, 10am-11:30am

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ADMIN Assignment #2 changed

A2Q1 moved to A3Q1

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OVERVIEW OF DATABASE DEVELOPMENT

Requirements Analysis / Ideas

High-Level Database Design

Conceptual Database Design / Relational Database Schema

Physical Database Design / Relational DBMS

Similar to software development

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Requirements Analysis What data are to be stored in the enterprise? What are the required applications? What are the most important operations?

High-level database design What are the entities and relationships in the

enterprise? What information about these entities and relationships

should we store in the database? What are the integrity constraints or business rules that

hold?

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Conceptual database design What data model to implement for the DBS?

E.g., relational data model Map the high-level design (e.g., ER diagram) to a

(conceptual) database schema of the chosen data model.

Physical database design What DBMS to use? What are the typical workloads of the DBS? Build indexes to support efficient query processing. What redesign of the conceptual database schema is

necessary from the point of view of efficient implementation?

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ENTITY-RELATIONSHIP MODEL

Short: ER model. A lot of similarities with other modeling languages

such as UML. Concepts

Entities / Entity sets, Attributes, Relationships/ Relationship sets, and Constraints.

Offers more modeling concepts than the relational data model (which only offers relations).

Closer to the way in which people think.

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ENTITY-RELATIONSHIP DIAGRAMS

An Entity-Relationship diagram (ER diagram) is a graph with nodes representing entity sets, attributes and relationship sets. Entity sets denoted by rectangles. Attributes denoted by ovals. Relationship sets denoted by diamonds. Edges (lines) connect entity sets to their attributes and

relationship sets to their entity sets.

lot

dname

budgetdid

sincename

Works_In DepartmentsEmployees

ssn

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ENTITIES AND ENTITY SETS Entity: Real-world object distinguishable from

other objects e.g. employee Miller. Entity can be physical or abstract object.

An entity is associated with the attributes describing its properties.

Attribute values are atomic e.g. strings, integer or real numbers. Contain a single piece of information

Full name? Age?

Entity set: A collection of similar entities. E.g., all employees.

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ENTITIES AND ENTITY SETS

All entities in an entity set have the same set of attributes. (At least, for the moment!)

Each entity set has a key, i.e. a minimal set of attributes to uniquely identify an entity of this set. Key attributes are underlined.

Each attribute has a domain, i.e. a set of all possible attribute values.

Employees

ssnname

age

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ENTITIES AND ENTITY SETS

A key must be unique across all possible (not just the current) entities of its set.

A key can consist of more than one attribute. There can be more than one key for a given

entity set, but we choose one (primary key) for the ER diagram.

Employees

firstnamelastname

birthdate

salary

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RELATIONSHIPS AND RELATIONSHIP SETS

Relationship: Association among two or more entities. E.g., Miller works in Pharmacy department.

Relationship set: Collection of similar relationships among two or more entity sets.

age

dname

budgetdid

name


ssn

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An n-ary relationship set R relates n entity sets E1 ... En.

Each relationship in R involves entities e1Î E1, ..., en Î En.

Binary relationship sets most common.

Same entity set can participate in different relationship sets, or in different “roles” in same set. Reports_To

age

name

Employees

subor-dinate

super-visor

ssn

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Entity object that is distinguishable from other objects Ex: your home address, CMPT 354

Entity Set All home addresses Collection of CMPT courses

Each entity set has 1-to-many entities Each entity can belong to multiple entity sets

Relationship Joe lives at 45 Main St. Mary lives at 89 Wood Ave.

Relationship Set Person lives at home address

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Relationship sets can also have attributes. Useful for properties that cannot reasonably be

associated with one of the participating entity sets.

age

dname

budgetdid

sincename


ssn

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INSTANCES OF AN ER DIAGRAM Entity set contains a set of entities. Each

entity has one value for each of its attributes. No duplicate instances

(not a technical limit) What to do??

ssn name age

12345678 “John Miller”

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14789632 “Paul Li” 25

. . . . . . . . .

Employees

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INSTANCES OF AN ER DIAGRAM Relationship set contains a set of

relationships, each relating a set of entities, one from each of the participating entity sets.

Components are entities, not attribute values. No duplicates

(not a technical limit)

Employee (ssn) Department (did)

12345678 1

14789632 1

56756322 2

. . . . . .

Works_In

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Multiway relationship sets (n > 2) are used whenever binary relationships cannot capture the application semantics.

TasksWorks_For

name

Employees

ssn age

Projects

pid pbudget

description

tid

Infrequent.

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Works_For

name

Employees

ssn age

Projects

pid pbudget

Employee (ssn)

Tasks (tid) Project (pid)

12345678 1000 101

12345678 1500 106

56756322 1500 106

. . . . . . . . .

Works_For

Tasks

descriptiontid

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KEY CONSTRAINTS A key constraint on a relationship set

specifies that the marked entity set participates in at most one relationship of this relationship set.

Entity set is marked with an arrow.

dname

budgetdid

since

age

name

ssn

ManagesEmployees Departments

Key constraint

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MULTIPLICITY OF RELATIONSHIPS An employee

can work in many departments; a dept can have many employees.

Each dept has at most one manager, who may manage several (many) departments.

dname

budgetdid

since

age

name

ssn

ManagesEmployees Departments

age

dname

budgetdid

sincename


ssn

manyone

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MULTIPLICITY OF RELATIONSHIPS The different types of (binary) relationships

from a multiplicity point of view: One to one One to many Many to one Many to many

many-to-manyone-to-one one-to-many many-to-one

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PARTICIPATION CONSTRAINTS

A participation constraint on a relationship set specifies that the marked entity set participates in at least one relationship of this relationship set.

Entity set is marked with a bold line.

age

name dnamebudgetdid

sincename dname

budgetdid

since

Manages

since

DepartmentsEmployees

ssn

Works_In

Participationconstraint

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WEAK ENTITIES A weak entity exists only in the context of another

(owner) entity. The weak entity can be identified uniquely only by

considering the primary key of the owner and its own partial key. Owner entity set and weak entity set must participate in a one-

to-many relationship set (one owner, many weak entities). Weak entity set must have total participation in this supporting

relationship set.

Ex: If there is no employee, there cannot be a dependent.

age

name

agename

DependentsEmployees

ssn

Policy

cost

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SUBCLASSES Sometimes, an entity set contains some

entities that do share many, but not all properties with the entity set hierarchies.

A ISA B: every A entity is also considered to be a B entity. A specializes B, B generalizes A.

A is called subclass, B is called superclass.

A subclass inherits the attributes of a superclass, may define additional attributes.

Contract_Emps

Employees

ISA

Hourly_Emps

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SUBCLASSES

Contract_Emps

namessn

Employees

age

hourly_wages

ISA

Hourly_Emps

contractid

hours_worked

Hourly_Emps and Contract_Emps inherit the ssn (key!), name and age attributes from Employees.

They define additional attributes hourly_wages, hours_worked and contractid, resp.

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SUBCLASSES

Covering constraints: Does every Employees entity have to be either an Hourly_Emps or a Contract_Emps entity?

NO. Unless Hourly_EmpsAND Contract_EmpsCOVER Employees

Overlap constraints: Can Joe be an Hourly_Emps as well as a Contract_Emps entity?

YES. Hourly_Emps OVERLAPS Contract_Emps

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SUBCLASSES

There are several good reasons for using ISA relationships and

subclasses: Do not have to redefine all the attributes. Can add descriptive attributes specific to a

subclass. To identify entitity sets that participate in a

relationship set as precisely as possible. ISA relationships form a tree

structure (taxonomy) with one entity set serving as root.

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DESIGN PRINCIPLES Faithfulness

Design must be faithful to the specification / reality.

Relevant aspects of reality must be represented in the model.

Avoiding redundancy Redundant representation blows up ER diagram

and makes it harder to understand. Redundant representation wastes storage. Redundancy may lead to inconsistencies in the

database.

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DESIGN PRINCIPLES Keep it simple

The simpler, the easier to understand for some (external) reader of the ER diagrams.

Avoid introducing more elements than necessary. If possible, prefer attributes over entity sets and

relationship sets. Formulate constraints as far as possible

A lot of data semantics can (and should) be captured.

But some constraints cannot be captured in ER diagrams.

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HIGH-LEVEL DESIGN WITH ER MODEL

Major design choices Should a concept be modeled as an entity or an

attribute? a relationship? What relationships to use: binary or ternary?

Should address be an attribute of Employees or an entity (connected to Employees by a relationship)?

Depends upon the use we want to make of address information, and the semantics of the data:

If we have several addresses per employee, address must be an entity (since attributes cannot be set-valued).

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ENTITY VS. ATTRIBUTE

Works_In2 does not allow an employee to work in the same department for two or more periods (why?).

We want to record several values of the descriptive attributes for each instance of this relationship.

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ENTITY VS. RELATIONSHIP

This ER diagram o.k. if a manager gets a separate discretionary budget for each dept.

But what if a manager gets a discretionary budget that covers all managed depts? Redundancy of dbudget, which is stored for each

dept managed by the manager. Misleading: suggests dbudget tied to managed

dept.

Manages2

name dnamebudgetdid

Employees Departments

ssn lot

dbudgetsince

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ENTITY VS. RELATIONSHIP

What about this diagram?

Employees who are not managers will have dbudget=null?

The following ER diagram is more appropriate and avoids the above problems!

Each manager now has a budget.

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BINARY VS. TERNARY RELATIONSHIPS

If each policy is owned by just one employee: Key constraint on Policies would mean policy can only

cover 1 dependent! (only 1 combination of Employees and Policies can be in Covers)

Bad design!

agepname

DependentsCovers

name

Employees

ssn lot

Policies

policyid cost

ER diagram says Employee can own several policies Each policy can be owned by several employees Each dependent can be covered by several policies

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This diagram is a better design. Policy can only exist for employees. Dependents

only exist if they are covered by a policy.

Beneficiary

agepname

Dependents

policyid cost

Policies

Purchaser

name

Employees

ssn lot

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Previous example illustrated a case when two binary relationships were better than one ternary relationship.

An example in the other direction: a ternary relation Contracts relates entity sets

Parts, Departments and Suppliers, and has descriptive attribute qty. No combination of binary relationships is an adequate substitute:

S “can-supply” P, D “needs” P, and D “deals-with” S does not imply that D has agreed to buy P from S.

How do we record qty?

D ATABASE S YSTEMS I W EEK 2 T HE E NTITY -R ELATIONSHIP M ODEL.

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