CMPT 258 Database SystemsThe Entity-Relationship ModelPart II (Chapter 2)

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Programming for Fun (PFF)When: Tuesday, September 22 3:30PMWhere: RLC 105Team based: one, two or three people per teamLanguages: C/C++ and JavaIDEs: Visual Studio, Eclipse, NetBeans

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Register your team online or in RLC 203 (Find the registration list on the bulletin board)Contact Dr. Tina Tian for questions.

ISA (‘is a’) Hierarchies• As in C++, or other PLs, attributes are inherited.• If we declare A ISA B, every A entity is also

considered to be a B entity.• Employees is specialized into subclasses• Hourly-Emp and Contract_EMps are generalized

by Employees

ISA (‘is a’) Hierarchies• Hourly-Emp and Contract_EMps are generalized

by Employees name

ssn

Employees

lot

Contract_Emps

hourly_wages

ISA

Hourly_Emps

contractid

hours_worked

ISA (‘is a’) Hierarchies• Overlap constraints: Can Joe be an Hourly_Emps

as well as a Contract_Emps entity? (Allowed/disallowed)

• Covering constraints: – Does every Employees entity also have to

be an Hourly_Emps or a Contract_Emps entity? (Yes/no).

ISA (‘is a’) Hierarchies

•Overlap constraints:▫The subclasses must have disjoint or overlapping sets

of entities

Disjoint(d) Overlap (0)

ISA (‘is a’) Hierarchies•Covering constraints (completeness

constraints): ▫An entity may not belong to any of the subclasses▫Every entity in the superclass must be a member of

some subclass

Partial Total

ISA (‘is a’) Hierarchies

•Multiple specializations

ISA (‘is a’) Hierarchies• Reasons for using ISA: – To add descriptive attributes specific to a

subclass.– i.e. not appropriate for all entities in the

superclass– To identify entities that participate in a

relationship. i.e., not all superclass entities participate

Aggregation• Suppose that we have an entity set called

Projects and that each Projects entity is sponsored by one or more departments.

• A department that sponsors a project might assign employees to monitor the sponsorship.

budgetdidpid

started_on

pbudgetdname

DepartmentsProjects Sponsors

since

NM

Aggregation

• Used to model a relationship between a collection of entities and relationships.

• Aggregation allows us to treat a relationship set as an entity set for purposes of participation in (other) relationships.

budgetdidpid

started_on

pbudgetdname

until

DepartmentsProjects Sponsors

Employees

Monitors

lotname

ssn

since

Aggregation• Aggregation vs. ternary

relationship: why not make Sponsors a ternary relationship?

• There are really two distinct relationships (monitors and sponsors), each with attribute of its own.

• Monitors is a distinct relationship, with a descriptive attribute.

budgetdidpid

started_on

pbudgetdname

until

DepartmentsProjects Sponsors

Employees

Monitors

lotname

ssn

since

Ternary Relationship

•Suppose that each department has offices in several locations and we want to record the locations at which each employee works.

Conceptual Design Using the ER Model•Design choices:

▫Should a concept be modeled as an entity or an attribute?

▫Should a concept be modeled as an entity or a relationship?

▫Identifying relationships: Binary or ternary? Aggregation?

•Constraints in the ER Model:▫A lot of data semantics can (and should) be

captured.

Entity vs. Attribute

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

Employees

ssnname

address

Entity vs. Attribute

• 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).

• If the structure (city, street, etc.) is important, e.g., we want to retrieve employees in a given city, address must be modeled as an entity (since attribute values are atomic).

Employees

ssnname

address

Entity vs. Attribute• If we have several addresses per employee,

address must be an entity (since attributes cannot be set-valued).

• If the structure (city, street, etc.) is important, e.g., we want to retrieve employees in a given city, address must be modeled as an entity (since attribute values are atomic).

street

citya#

name

ssn

Lives_inEmployees addresses

Entity vs. Attribute

• Does Works_In4 allow an employee to work in a department for two or more periods?

name

Employees

ssn lot

Works_In4

from todname

budgetdid

Departments

Entity vs. Attribute• Similar to the problem of wanting to record several

addresses for an employee: – We want to record several values of the

descriptive attributes for each instance of this relationship.

– Accomplished by introducing new entity set, Duration.

name

Employees

ssn lot

Works_In4

from todname

budgetdid

Departments

dnamebudgetdid

name

Departments

ssn lot

Employees Works_In4

Durationfrom to

name

Employees

ssn lot

Works_In4

from todname

budgetdid

Departments

Entity vs. Attribute

Entity vs. Relationship• Scenario 1: a manager gets a separate

discretionary budget for each department.

• Scenario 2: What if a manager gets a discretionary budget (sum) that covers all managed departments?

Manages2

name dnamebudgetdid

Employees Departments

ssn lot

dbudgetsince

Entity vs. Relationship• What if a manager gets a discretionary budget

(sum) that covers all managed departments?– Redundancy: dbudget stored for each dept

managed by manager.– Misleading: Suggests that dbudget associated

with department-mgr combination.

Manages2

name dname

budgetdid

Employees Departments

ssn lot

dbudgetsince

Entity vs. Relationship

•This fixes the problem!

dname

budgetdid

DepartmentsManages2

Employees

namessn lot

since

Managers dbudget

ISA

Binary vs. Ternary Relationships • An example of ternary relationship :

▫a ternary relation Contracts relates entity sets Parts, Departments and Suppliers, and has descriptive attribute qty.

Suppliers

qty

DepartmentsContractParts

Binary vs. Ternary Relationships • 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?

Suppliers

qty

DepartmentsContractParts

Suppliers

Departments

deals-with

Parts

can-supply

needs

VS.

Summary of Conceptual Design• Conceptual design follows requirements analysis• ER model popular for conceptual design

▫Constructs are expressive, close to the way people think about their applications.

▫There are many variations on ER model (both graphically and conceptually)

• Basic constructs ▫entities, relationships, and attributes (of entities

and relationships)• Some additional constructs

▫weak entities, ISA hierarchies, and aggregation

Summary of Conceptual Design• Several kinds of integrity constraints:

▫key constraints▫participation constraints▫overlap/covering for ISA hierarchies.

• Constraints play an important role in determining the best database design for an enterprise.

Summary of Conceptual Design• ER design is subjective. There are often many ways

to model a given scenario! • Analyzing alternatives can be tricky, especially for a

large enterprise. Common choices include: Entity vs. attribute, entity vs. relationship,

binary or n-ary relationship, whether or not to use ISA hierarchies, and whether or not to use aggregation.

• Ensuring good database design: resulting relational schema should be analyzed and refined further. ▫ FD (functional dependency) information and

normalization techniques are especially useful.

Homework 1

•Cover page with name and homework number

•Hard copy preferred•Due Sep 24 (Thur) before class

Readings

•Chapter 3