1 cse 480: database systems lecture 2: entity-relationship modeling reference: read chapter 3 of the...

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CSE 480: Database Systems

Lecture 2: Entity-Relationship Modeling

Reference:

Read Chapter 3 of the textbook

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Database Design

Goal is to derive a specification of the database schema– Schema is the description of the database (e.g., names of tables,

columns/attributes, attribute types, and constraints)

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Database Design

4 key steps: – Requirement analysisRequirement analysis:

Discover what information needs to be stored and how the stored information will be used

– Conceptual database design:Conceptual database design: Create conceptual schema for the database using high-level data

model (e.g., entity-relationship modeling)

– Logical database designLogical database design: Convert E-R model to implementation data model (relational model)

– Physical designPhysical design: Specify the internal storage structure, indexes, and file organizations

for the database files

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Example: COMPANY Database

Requirement analysis:– The company is organized into DEPARTMENTs.

– Each department has a unique name, unique number and an employee who manages the department. We keep track of the start date of the department manager. A department may have several locations.

– Each department controls a number of PROJECTs. Each project has a unique name, unique number and is located at a single location

– We store each EMPLOYEE’s social security number, address, salary, sex, and birthdate. Each employee works for one department but may work on several projects. We need to keep track of the number of hours per week that an employee currently works on each project. We also keep track of the direct supervisor of each employee.

– Each employee may have a number of DEPENDENTs. For each dependent, we keep track of their name, sex, birthdate, and relationship to employee

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Conceptual Design: E-R Diagram

We will explain how to construct such a

diagram in the next two lectures

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Logical Design: Mapping to Relational Schema

We will explain how to map the E-R diagram to a relational schema

in lecture 7

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Entity-Relationship (E-R) Diagram

A design methodology for modeling the concepts in an enterprise (mini-world)

Concepts:– Entity types

– Relationship types

– Constraints

E-R Diagrams provide a graphical representation of the entities, relationships, and constraints that make up a given design

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Entity Types: The ‘E’ in E-R Diagram

EntitiesEntities: specific “objects” or “things” in the mini-world that are represented in the database

– Professor John Doe, Electrical Engineering Department, CSE480, the red car that always park next to the building entrance, etc

Entity TypeEntity Type: collection of similar entities– Bob Doe and Mary Doe are STUDENTs

– Electrical Engineering is a DEPARTMENT

E-R diagram models the entity types (not individual entities)

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Attributes of Entity Types

AttributesAttributes are properties associated with an entity type– Attributes of EMPLOYEE entity type include Name, SSN,

Employee ID, BirthDate, Address, Salary, StartDate, etc

– Attributes of STUDENT entity type include Name, PID, GPA, sex, major, last semester enrolled, etc.

When designing the E-R diagram, you need to– List all the entity types

– List the attributes associated with each entity type You also need to know the TYPE of each attribute

– Simple or composite

– Single-valued or multi-valued

– Stored or derived

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Types of Attributes

Simple (Atomic) vs Composite– Simple (atomic) attributes are indivisible

SSN, Gender, Salary, …

– Composite attributes may be composed of several components Name (FirstName, MiddleName, LastName) May have nested components

Address( Street_address (Number, Street, Apartment_number), City, State, Zip)

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Types of Attributes

Single-valued vs. Multi-valued– Single-valued: one value for each entity

Examples: Age, Birth Date, SSN

– Multi-valued: Multiple values for each entity Examples: Colors of a CAR, Hobbies of a STUDENT, Email

addresses of a PERSON

– Multi-valued composite (Complex) attribute Ex: PreviousDegrees of a STUDENT

denoted by {PreviousDegrees (College, Year, Degree, Field)}

– Ex: {(MSU, 1994, BS, CS), (UM, 1996, MS, CS)}

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Types of Attributes

Stored vs Derived– Derived attribute is not physically stored in the database; its

value is computed from other attributes or from related entities Examples:

– Age (derived from Birth Date),

– NumberOfEmployees (derived by counting number of entities associated with the Employee entity type),

– GPA (derived by averaging the grades of each STUDENT entity from the GRADE_REPORT entity type)

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Example: MOVIE Entity Type

MOVIE entity type has the following attributes

Attribute Simple Composite Single-valued

Multi-valued

Stored Derived

Title

Release_date

Director

Genre

#Awards

Production_company

Attribute Simple Composite Single-valued

Multi-valued

Stored Derived

Title

Release_date

Director

Genre

#Awards

Production_company

X X X

X X X

X X X

X X X

X X X

X X X

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Constraints on Entity Types

When designing the E-R diagram, you also need to think about constraints on the entity types

– Domain constraint

– Null constraint

– Key constraint

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Domain Constraint

Each attribute is associated with a set of values(domain or data type)

– Employee ID is CHAR(10),

– Salary is FLOAT,

– StartDate is DATE,

– SSN is CHAR(10)

– Hourly is BOOLEAN (Hourly employee vs Salaried employee)

The domain of an attribute restricts the range of valid values an attribute can have (domain constraint)

– Example: if SSN is CHAR(10) and you try to add a record with SSN = “123-456-1211” to the database, it will violate the domain constraint of the attribute (so the DBMS will throw an error)

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NULL Constraint

A special placeholder to denote the following:– When an attribute is inapplicable to an entity

– When an attribute value is unknown or missing

Not exactly a “value”– If John’s blood type is NULL and Mary’s blood type is NULL, it

does not mean that they both have the same attribute values

Null constraint: restricts whether an attribute value can be NULL

ID Name Weight Blood Pressure Blood Type

1 John Smith 160 NULL NULL

2 Mary Smith NULL 110/75 NULL

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Key Constraint

When you store a data instance (record) into the database, you expect to be able to retrieve it with a query

– To do this, you will need to distinguish each data instance from other instances in the database

Key attribute: attribute for which each entity must have unique value

– Examples: SSN of EMPLOYEE, PID of STUDENT, DEPTNUMBER of DEPARTMENT

Key constraint: Prohibits two entities from having the same value for the key attribute

– Used to uniquely identify individual entities in a database

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Key Constraint

A key attribute may be composite– Registration is a key of CAR entity type with components

(Registration_State, Registration_Number)

Minimality property: superfluous attribute must not be included in the key

– Ex: SSN is minimal whereas (SSN, Name) is not minimal

Some entity types may have more than one key– Ex: VehicleID and (Registration_State, Registration_Number)

are keys to CAR

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Representing Entity Type in E-R Diagram

Multi-valued attributes

have double ovals

Composite attribute

Entity typeAttribute

Key attributes

are underlined

Key attributes

are underlined

No_Owners

Derived attribute

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Summary

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Exercise: COMPANY database

Requirements:– The company is organized into DEPARTMENTs.


– Each department controls a number of PROJECTs. Each project has a unique name, unique number and is located at a single location

– We store each EMPLOYEE’s social security number, address, salary, sex, and birthdate. Each employee works for one department but may work on several projects. We keep track of the number of hours per week that an employee currently works on each project. We also keep track of the direct supervisor of each employee.


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Requirements:– The company is organized into DEPARTMENTs.


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Requirements:– Each department controls a number of PROJECTs. Each project

has a unique name, unique number and is located at a single location

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Requirements:

– We store each EMPLOYEE’s social security number, address, salary, sex, and birthdate. Each employee works for one department but may work on several projects. We keep track of the number of hours per week that an employee currently works on each project. We also keep track of the direct supervisor of each employee.

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Requirements:


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Entity Types

This initial design is not complete

Entity types are not independent

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Refining design by using relationships

Relationships between entity types

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Relationships and Relationship Types

Relationship: relates two or more entities– EMPLOYEE John Doe works for Geography DEPARTMENT

– EMPLOYEE Mary Smith works for Chemistry DEPARTMENT

Relationships of the same type are grouped into a relationship type

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Representing Relationship Types in ER Diagram

PROJECTDEPARTMENT CONTROLS

(Draw the rest of its attributes)

LocationName

Number

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Attributes of a Relationship Type

EMPLOYEE DEPARTMENTMANAGES

Start_date Name Number

Locations


Relationship types can also have attributes

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Relationship Types for COMPANY database

EMPLOYEE

PROJECT

WORKS_ON

Hours

WORKS_FOR

DEPARTMENTSUPERVISION

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Relationship Types for COMPANY database

Not quite right yet! (We will revisit this in lecture 3)


DEPENDENT EMPLOYEEDEPENDENTS

_OF(Draw the rest of

its attributes)

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Recursive Relationships and Roles

Relationship can relate elements of same entity type (recursive relationship)

– Ex: Supervises relationship type relates two Employee entity types Mary supervises Bob

– Need to distinguish different entities participating in a relationship

SUPERVISION

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Roles

Use role name to indicate the role that a participating entity plays in a relationship instance

– SUPERVISION has roles Subordinate and Supervisor

– Role names must be provided for every recursive relationship type

– Role names are not necessary where all participating entity types are distinct

SUPERVISION

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Relationship Degree

Degree: the number of entity types participating in a relationship type

– Binary relationships (WORKS_ON, MANAGES)

– Ternary relationship

SUPPLIER PROJECTSUPPLY

PART

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Summary

Conceptual database design– Using E-R modeling

Entity types

– Domain, null, and key constraints

Relationship types, their attributes, roles, and degree

– Relationship Constraints? (next lecture)

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Exercise

Choose a domain, for example:– Airline reservation system

– Electronic medical records

– Online bookstore (e.g., Amazon)

– Law enforcement (e.g., FBI criminal database)

– Online photo sharing (e.g., Flickr)

– College football/basketball database

Answer the following questions:– What are the entity types and their corresponding attributes?

– What are the relationship types and their corresponding attributes?

– What are the constraints? Are these constraints on entity types or relationship types?

– Is there any other constraints that cannot be easily modeled?

– Draw the E-R diagram

1 cse 480: database systems lecture 2: entity-relationship modeling reference: read chapter 3 of the...

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