CSC 411/511: DBMS Design 1 1 Dr. Nan WangCSC411_L2_ER Model 1 The Entity-Relationship Model (Chapter...

CSC 411/511: DBMS Design

1

Dr. Nan Wang CSC411_L2_ER Model1

The Entity-Relationship Model

(Chapter 2)

CSC411_L2_ER ModelDr. Nan Wang22

Main Phases of Database Design


Overview of Database Design

• Conceptual design: (ER Model is used at this stage.) – 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? – A database `schema’ in the ER Model can be represented

pictorially (ER diagrams).– Can map an ER diagram into a relational schema.


ER Model Basics

• Entity-Relationship (ER) data model – A database can be modeled as a collection of entities and

relationship among entities– Widely used to develop an initial database design

• Entity: Real-world object distinguishable from other objects.

– An entity is described (in DB) using a set of attributes.

Employees

ssnname

lot


ER Model Basics (Contd.)

• Entity Set: A collection of similar entities. E.g., all employees. – All entities in an entity set have the same set of attributes.

– Each entity set has a key. • Key: a minimal set of attributes whose values uniquely identify

an entity in the set

– Each attribute has a domain.• A domain defines the possible values of each attributes

• E.g., attribute name might be the set of 20-character string

Employees

ssnname

lot




• Relationship: Association among two or more entities. – E.g., James works in Pharmacy department.– Descriptive attribute, since– Record the information about the relationship, rather than

about one of the participating entities

did

dname

budget

Departments

lot

sincename

Works_InEmployees

ssn



• Relationship Set: Collection of similar relationships.

• Same entity set could participate in different relationship sets

lot

name dnamebudgetdid

sincename dname

budgetdid

since

Manages

since

DepartmentsEmployees

ssn

Works_In



• Relationship Set: Collection of similar relationships.

• A relationship might involve two entities in the same entity set

– Two entities with different “roles” in same set.

subor-dinate

super-visor

Reports_To

lot

name

Employees

ssn


Key Constraints

• Consider Works_In: An employee can work in many departments; a dept can have many employees.

Many-to-Many1-to-1 1-to Many Many-to-1

did

dname

budget

Departments

lot

sincename

Works_InEmployees

ssn


Key Constraints

• In contrast, each dept has at most one manager, according to the key constraint on Manages.

• The constraint that each department has at most one manager is an example of a key constraint, and it implies that each Departments entity appear in at most one Manages relationship.

• Arrow states that given a Department entity, we can uniquely determine the manages relationship in which it appears.


dname

budgetdid

since

lot

name

ssn

ManagesEmployees Departments


Key Constraints

• Each department has only one manager

• And each employee can manage only one department.

12


dname

budgetdid

since

lot

name

ssn

ManagesEmployees Departments


Participation Constraints

• Does every department have a manager?– If so, this is a participation constraint: the participation of

Departments in Manages is said to be total (vs. partial).• If the participation of an entity set in a relationship set is total,

the two are connected by a thick line• Every Departments entity must appear in an instance of the

Manages relationship.

lot

name dnamebudgetdid

sincename dname

budgetdid

since

Manages

since

DepartmentsEmployees

ssn

Works_In


exercise

14






Homework

• Page 53, exercise 2.4

• Submit your homework (softcopy, PDF or DOC format) at [email protected] or hardcopy at class)

• Due date: Next Thursday (Sept, 5th)

19


Weak Entities

• Employees can purchase insurance to cover their dependents.

• Only dependents’ SSN is not needed• If an employee quits, any policy owned by the

employee is terminated and all relevant policy and dependent info will be deleted from the database.

20

lot

name

agepname

DependentsEmployees

ssn

Policy

cost

ssn


Weak Entities

• A weak entity can be identified uniquely only by considering the primary key of another (owner) entity.– (One of two or more candidate keys is designated as the primary key)– (Partial key: the set of attributes of a weak entity set that uniquely

identify a weak entity for a given owner entity)– 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 identifying

relationship set.

lot

name

agepname

DependentsEmployees

ssn

Policy

cost


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

namessn

Employees

lot

Contract_Emps

hourly_wagesISA

Hourly_Emps

contractid

hours_worked


ISA (`is a’) Hierarchies (Contd.)

• 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).

Intuitively, No.

• Reasons for using ISA: – To add descriptive attributes specific to a subclass.

– To identify entities that participate in a relationship.


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 (Contd.)

• 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.

– Also, can say that each sponsorship is monitored by at most one employee.

– How to set key constraint in aggregation and ternary relationship?

budgetdidpid

started_on

pbudgetdname

until

DepartmentsProjects Sponsors

Employees

Monitors

lotname

ssn

since


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.– But some constraints cannot be captured in ER diagrams.


Entity vs. Attribute

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

Employees

ssnname

address



• 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



street

citya#

name

ssn

Lives_inEmployees addresses

•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).


Entity vs. Attribute (Contd.)

name

Employees

ssn lot

Works_In4

from todname

budgetdid

Departments

diddname

budget

Departments

lot

sincename

Works_InEmployees

ssn


Entity vs. Attribute (Contd.)

• Works_In4 does not allow an employee to work in a department for two or more periods.

• 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.

dnamebudgetdid

name

Departments

ssn lot

Employees Works_In4

Durationfrom to

name

Employees

ssn lot

Works_In4

from todname

budgetdid

Departments


Entity vs. Relationship

• First ER diagram OK if a manager gets a separate discretionary budget for each dept.

• What if a manager gets a discretionary budget (sum) that covers all managed depts?

– Redundancy: dbudget stored for each dept managed by manager.

– Misleading: Suggests that dbudget associated with department-mgr combination.

Manages2

name dnamebudgetdid

Employees Departments

ssn lot

dbudgetsince

dnamebudgetdid

DepartmentsManages2

Employees

namessn lot

since

Managers dbudget

ISA

This fixes theproblem!


Binary vs. Ternary Relationships (1)

• The ER diagram shows– An employee can own several policies (dental, health)– Each policy can be owned by several employees– Each dependent can be covered by several policies

agepname

DependentsCovers

name

Employees

ssn lot

Policies

policyid cost

lot

name

agepname

DependentsEmployees

ssn

Policy

cost



• If each policy is owned by just 1 employee, and each dependent is tied to the covering policy, this ER diagram is inaccurate. Why?– Key constraint on Policies with respect to Covers implies that a policy can

cover only one dependent.

agepname

DependentsCovers

name

Employees

ssn lot

Policies

policyid cost



• Additional constraints in the following ER diagram– A policy cannot be owned jointly by two or more employees (key constraint)– Every policy must be owned by some employees (total participation

constraint)– Dependents is a week entity set

• (uniquely identified by pname and policyid)

Beneficiary

agepname

Dependents

policyid cost

Policies

Purchaser

name

Employees

ssn lot

Better design



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

• An example of ternary relationship – 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 (without aggregation)

• 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?


Summary of Conceptual Design

• Conceptual design follows requirements analysis, – Yields a high-level description of data to be stored

• ER model popular for conceptual design– Constructs are expressive, close to the way people think

about their applications.• Basic constructs

– entities, relationships, and attributes (of entities and relationships).

• Some additional constructs – weak entities, ISA hierarchies, and aggregation.

• Note: There are many variations on ER model.


Summary of ER (Contd.)

• Several kinds of integrity constraints can be expressed in the ER model – key constraints, participation constraints, and

overlap/covering constraints for ISA hierarchies. – Some foreign key constraints are also implicit in the

definition of a relationship set.• Some constraints (notably, functional

dependencies) cannot be expressed in the ER model.

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


Summary of ER (Contd.)

• 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.




CSC 411/511: DBMS Design

43

Dr. Nan Wang CSC411_L2_ER Model43

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