1 The Entity-Relationship Model 2. 2 Database Design Process Requirement collection and analysis ...

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The Entity-Relationship Model

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Database Design Process

Requirement collection and analysis DB requirements and functional requirements

Conceptual DB design using a high-level model Easier to understand and communicate with others

Logical DB design (data model mapping) Conceptual schema is transformed from a high-

level data model into implementation data model Physical DB design

Internal data structures and file organizations for DB are specified

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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). An ER diagram can be mapped into a relational

schema.

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Relational Model [Properties] Each relation (or table) in a database

has a unique name An entry at the intersection of each row

and column is atomic (or single-valued);there can be no multi-valued attributes in a relation

Each row is unique; no two rows in a relation are identical

Each attribute (or column) within a table has a unique name

The Relational Model

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Properties Cont’d The sequence of columns (left to

right) is insignificant; the columns of a relation can be interchanged without changing the meaning or use of the relation

The sequence of rows (top to bottom) is insignificant;rows of a relation may be interchanged or stored in any sequence

The Relational Model

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The Relational Model...The relational model of data has

three major components: Relational database objects

allows to define data structures

Relational operators allows manipulation of stored data

Relational integrity constraints allows to defines business rules and ensure data integrity

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The Relational Objects Location

Most RDBMS can have multiplelocations, all managed by the same database engine

Marketing Purchasing

AccountingMarketing

Sales Advertising

Accounting

Accounts Receivable

Accounts Payable

Corporate Database

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Database Server

Multi-user

Client Applications

The Relational Objects

Location

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The Relational Objects...Database

A set of SQL objects

UPDATE UPDATE TT SET SETINSERT INTO INSERT INTO TTDELETE FROM DELETE FROM TTCALL STPROGCALL STPROG

Client Application

Database Server

StoredProcedure

BEGIN...

Table A

Table B

Table T

Update Trigger

Insert Trigger

Delete Trigger

BEGIN...

BEGIN...

BEGIN...

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The Relational Objects...Database

A collection of tables and associated indexes

Table

Department

Table

Product

Table

Customer

Table

Employee

Index

Files

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The Relational Objects...Relation

A named, two dimensional table of data

Database A collection of databases, tables and

related objects organised in a structured fashion.

Several database vendors use schema interchangeably with database

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Relational Objects...

Tables are comprised of rows and a fixed number of named columns.

Data is presented to the user as tables:

Column 1 Column 2 Column 3 Column 4

Row

Row

Row

Table

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Columns are attributes describing an entity. Each column must have an unique name and a data type.


Name Designation Department

Row

Row

Row

Employee

Structure of a relation (e.g. Employee)Employee(Name, Designation, Department)

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Rows are records that present information about a particular entity occurrence


Name Designation Department

Row

Row

Row

Employee

De Silva Manager Personnel

Perera Secretary Personnel

Dias Manager Sales

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Relational model terminology

Row is called a ‘tuple’ Column header is called an ‘attribute’ Table is called a ‘relation’ The data type describing the type of values that

can appear in each column is called a ‘domain’ Eg:-

Names : the set of names of persons Employee_ages : value between 15 & 80 years oldThe above is called ‘logical definitions of domains’.A data type or format can also be specified for each

domain.Eg: The employee age is an integer between 15 and 80

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Characteristics of relations

Ordering of tuples Tuples in a realtion don’t have any particular order.

How ever in a file they may be physically ordered based on a criteria, this is not there in relational model

Ordering of values within tuple Ordering of values within a tuple are unnecessary,

hence a tuple can be considered as a ‘set’. But when relation is implemented as a file attributes

may be physically ordered Values in a tuple are atomic

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Relational constraints

Domain constraints specifies that the value of each attribute ‘A’ must be

an atomic value. And from the specified domain Key constraints

There is a sub set of attributes of a relational schema with the property that no two tuples should have the same combination of values for the attributes.

Any such subset of attributes is called a ‘superkey’ A ‘superkey’ can have redundant attributes. A key is

a minimul superkey If a realtion has more than one key, they are called

candidate keys One of them is chosen as the primary key

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Relational Objects

Primary Key: An attribute (or combination of attributes) that uniquely identifies each row in a relation.Employee(Emp_No, Emp_Name, Department)

Composite Key: A primary key that consists of more than one attributeSalary(Emp_No, Eff_Date, Amount)

Keys

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Relational Objects

Each table has a primary key. The primary key is a column or combination of columns that uniquely identify each row of the table.


Primary Key

EmployeeE-No E-Name D-No

179 Silva 7857 Perera 4342 Dias 7

Primary Key

SalaryE-No Eff-Date Amt

179 1/1/98 8000857 3/7/94 9000179 1/6/97 7000342 28/1/97 7500

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179 1/1/98 8000857 3/7/94 9000179 1/6/97 7000342 28/1/97 7500

Relational Objects

The cardinality of a table refers to the number of rows in the table. The degree of a table refers to the number of columns.


Salary TableDegree = 3Cardinality = 4

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Entity integrity, referential integrity/foreign keys

Entity integrity constraint specifies that no primary key can be null

The referential integrity constraint is specified between two relations and is used to maintain the consistency among tuples of the two realtions

Informally what this means is that a tuple in one relation that refers to another relation must refer to an existing tuple.

To define referential integrity we use the concept of foreign keys.

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Relational Objects

Foreign Key: An attribute in a relation of a database that serves as the primary key of another relation in the same database

Employee(Emp_No, Emp_Name, Department)

Department(Dept_No, Dept_Name, M_No)

Relationship

=== works for ==>

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Relational Objects

A foreign key is a set of columns in one table that serve as the primary key in another table


Foreign KeyPrimary Key

Primary Key

D-No D-Name M-No

4 Finance 857 7 Sales 179

Primary Key

DepartmentEmployeeE-No E-Name D-No


Recursive foreign key: A foreign key in a relation that references the primary key values of that same relation

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Primary Key

D-No D-Name M-No

4 Finance 857 7 Sales 179

Primary Key

Department

EmployeeE-No E-Name D-No



Rows in one or more tables are associated with each other solely through data values in columns (no pointers).

Primary Key

Foreign KeyPrimary Key

Foreign Key

Foreign Key


179 1/1/98 8000857 3/7/94 9000179 1/6/97 7000342 28/1/97 7500

Primary Key

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Relational ObjectsIndex

An ordered set of pointers to the data in the table

E-No E-Name D-No

179 Silva 7857 Perera 4342 Dias 7719 De Silva 5

EmployeeE-Name Pointer

De Silva Dias Perera Silva

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Index: Employee NameE-No E-Name D-No

179 Silva 7857 Perera 4342 Dias 7719 De Silva 5587 Alwis 4432 Costa 6197 Zoysa 2875 Peiris 4324 Vaas 7917 Bandara 3785 Opatha 2234 Wickrama 1

EmployeeE-Name Pointer

Alwis Bandara Costa De Silva Dias Opatha Peiris Perera Silva Vaas Wickrama Zoysa

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E-Name Pointer

Alwis Bandara Costa De Silva Dias Opatha Peiris Perera Silva Vaas Wickrama Zoysa

Search: Employee Dias

Index

Improvesperformance.Access to data is faster

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Opatha

SilvaCosta

Bandara PereraDias Wickrama

Index

Ensures uniqueness. A table with unique fields in the index cannot havetwo rows with the same values in the column or columns that form the index key.

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. Opatha . .. Bandara . .

. Alwis . . . Dias . .. Costa . . . Peiris . . . Silva . . . Wickrama . Zoysa .

. De Silva . Perera .

. Vaas . .

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Relational Database

STOREStore 1 || ColomboStore 2 || Kandy

INVENTORYStore 1 || P1 || 50Store 1 || P3 || 20Store 2 || P2 || 100Store 2 || P1 || 30

ORDERSStore 1 || P3 || 3428 || 0052 || 10Store 2 || P2 || 3428 || 0098 || 7Store 2 || P3 || 3428 || 0098 || 15Store 2 || P4 || 5726 || 0099 || 1

PARTP1 | PrinterP2 | DisketteP3 | Disk DriveP4 | Modem

VENDOR3428 || East West5726 || DMS

STOREStore Name || City

INVENTORYStore Name || Part No || Quantity

ORDERSStore Name || Part No || Vendor No || Order No || Quantity

PARTPart No || Description

VENDORVendor No || Vendor Name

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ER Model Basics

Entity: Real-world object distinguishable from other objects. An entity is described (in DB) using a set of attributes.

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

attributes. (Until we consider ISA hierarchies, anyway!)

Each entity set has a key. Each attribute has a domain.

Employees

ssnname

lot

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ER Model Basics

Key and key attributes: Key: a unique value for an entity Key attributes: a group of one or more attributes

that uniquely identify an entity in the entity set Super key, candidate key, and primary key

Super key: a set of attributes that allows to identify and entity uniquely in the entity set

Candidate key: minimal super key• There can be many candidate keys

Primary key: a candidate key chosen by the designer

• Denoted by underlining in ER attributes

Employees

ssnname

lot

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ER Model Basics (Contd.)

Relationship: Association among two or more entities. e.g., Jack works in Pharmacy department.

Relationship Set: Collection of similar relationships. An n-ary relationship set R relates n entity sets E1 ... En;

each relationship in R involves entities e1 in E1, ..., en in En• Same entity set could participate in different relationship

sets, or in different “roles” in same set.

lot

dname

budgetdid

sincename

Works_In DepartmentsEmployees

ssn

Reports_To

lot

name

Employees

subor-dinate

super-visor

ssn

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Key Constraints

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

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

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

dname

budgetdid

since

lot

name

ssn

ManagesEmployees Departments

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Example ER• An ER diagram

represents several assertions about the real world. What are they?

• When attributes are added, more assertions are made.

• How can we ensure they are correct?

• A DB is judged correct if it captures ER diagram correctly.

Students

Professor teaches

Department

faculty

major offers

Courses

enrollmentadvisor

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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).

• 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

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Weak Entities A weak entity can be identified uniquely only by

considering the primary key of another (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

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ISA (`is a’) Hierarchies

Contract_Emps

namessn

Employees

lot

hourly_wagesISA

Hourly_Emps

contractid

hours_worked 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. Overlap constraints: Can Joe be an Hourly_Emps as well

as a Contract_Emps entity? (default: disallowed; A overlaps B)

Covering constraints: Does every Employees entity also have to be an Hourly_Emps or a Contract_Emps entity? (default: no; A AND B COVER C)

Reasons for using ISA: To add descriptive attributes specific to a subclass. To identify entities that participate in a relationship.

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Aggregation Used when we

have to model a relationship involving (entitity sets and) a relationship set. Aggregation

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

Aggregation vs. ternary relationship: Monitors is a distinct relationship, with a descriptive attribute. Also, can say that each sponsorship is monitored by at most one employee.

budgetdidpid

started_on

pbudgetdname

until

DepartmentsProjects Sponsors

Employees

Monitors

lotname

ssn

since

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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.

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Entity vs. Attribute

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).

• 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).

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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.

name

Employees

ssn lot

Works_In4

from todname

budgetdid

Departments

dnamebudgetdid

name

Departments

ssn lot

Employees Works_In4

Durationfrom to

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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 that covers all managed depts? Redundancy: dbudget

stored for each dept managed by manager.

Misleading: Suggests 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!

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Binary vs. Ternary Relationships

If each policy is owned by just 1 employee, and each dependent is tied to the covering policy, first diagram is inaccurate.

What are the additional constraints in the 2nd diagram?

agepname

DependentsCovers

name

Employees

ssn lot

Policies

policyid cost

Beneficiary

agepname

Dependents

policyid cost

Policies

Purchaser

name

Employees

ssn lot

Bad design

Better design

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Binary vs. Ternary Relationships (Contd.)

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?

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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.

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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.

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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 information and normalization techniques are especially useful.

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