entity-relationship diagrams

Entity-Relationship Diagrams

Solutions to Homework #1

Project Partners, Inc.Data Flow Diagram

Client

0 Billing & Reporting Consultant

Time Sheet & Expense Report

Invoice

Payment

Project Manager

Project Assignment Data

Level 0 DFD -- Context Diagram

Client & Project Data

Correction Request: Missing & Invalid Data


Client

5 Prepare Invoice

Consultant Time Sheet Data

Invoice

Payment Data

Project Manager

Client & Project Data1

Record Project Information

Client & ProjectData

Client & Project Data

2 Update Project Assignments

Project Assignment Data: (proj., consultant, dates, rate) Project

AssignmentProject Assignment Data

3 Record & Validate

Timesheets

Time LogTime Sheet Data

Assignment Data

Time Sheet Data

InvoiceInvoice Data

4 Record Expense

Data

Expense Reports

Expense Report

Expense Report Data

6 Receive Payment Invoice Status

Project Expense Data

Level 1 DFD -- Billing & Reporting System

Invoice Balance

Client

Client & Project Id

Correction Request: Missing & Invalid Data


ConsultantTimesheet Activity Data

3.1 Capture &

Record Activity Data

Time LogTimesheet Activity Data

3.2 Validate

TimesheetsProject

AssignmentProject Assignments

Time Log

Entries

Client & ProjectData

Client & Project Id

Correction Request

Employee Data

Employee IdActivity

Activity Data

3.3 Find Missing Timesheets

Timesheet Request

Employee Data

Current Employees

Level 2 DFD -- Record & Validate Timesheets

Current Time Log Entries

Time Log Validation

Status

Project Partners, Inc.Process Details

Process 3.1 -- Capture & Record Timesheet ActivityRecord the Timesheet records as prepared by the consultants. Save this information in the TimeLog data store. Also, make sure the status of each Time Log record indicates it has not yetbeen validated.

Note that there should be a separate process which ensures that each consultant provided timerecords for each day that they are employed (Process 3.3).


Process 3.2 -- Validate TimesheetsRead Employee record to validate Employee ID.Read Client record to validate Client ID.Read Project record to validate Project ID.Read Activity record to validate Activity ID.IF the Activity indicates it is an overhead activity, Ensure that the Timesheet indicates it is non-billable.IF Timesheet indicates a billable time record, Read the Project Assignment record to verify the consultant can bill against this project and the Activity indicates it can be billable and the Timesheet date should be within the approved work dates, and the Project Assignment should indicate it is billable.IF the total hours for a day is less than 8, Request the consultant to account for remaining time (alternatively, check for the total hours for a week being < 40).IF the total hours for a day is greater than 10 (or some other appropriate reasonable number), Verify with the consultant that the total is correct.IF each time log entry has pass all validations, Update the Tiime Log status for each entry indicating it is OK.


Process 3.3 -- Find Missing TimesheetsFor a time period for which timesheets are collected (e.g., each week), find the consultants whohave not turned in a Timesheet. This action can be performed by:1. Obtain a list of all individuals required to file timesheets (from the Employee data store);2. Obtain a list of all individuals who have filed timesheets (from the Time Log data store);3. Match the 2 lists, discarding those individuals who appear on both lists. The remainingpeople from the Employee data store have missing Timesheets.

Project PartnersInformation Requirements

• Consider this list to be the minimum set of data.• Client information (Company name, address)• Project name and description.

– This includes the starting and ending dates of the engagement; identifier of the underlying contract and an indication of the type of engagement (I.e., fixed price vs time & materials); and the client’s project manager information (their name, address, phone, and fax).

• List of PPI’s standard project activities– Each of these activities includes its activity id, description, and an

indicator of whether it is normally billable. Activities which are normally billable include analysis, design, programming, etc. Some activities (e.g., proposal preparation, vacation, sick time, etc.) are always considered to be overhead and hence are never billable.


• List of PPI’s standard project activities– Note that it is PPI’s policy to “charge” anything to a project that can be attributable

to a project or engagement, irrespective of whether it is billable and therefore to be included on an invoice. Please note that these are not specific instances of consultant’s activities, but a list of possible activities that could be entered by someone using the time sheet entry system.

• Standard billing rate for each class of consultant– Other information related to the class of consultant should include a title and brief

description.• Assignment information

– Each consultant who can record billable time for a project should have information pertaining to the beginning and ending dates of their billable involvement along with their actual billing rate for the project. Non-billable time may be charged by the consultants or by other individuals to a given project. This insures that all time spent on a project is recorded even if it is never included in an invoice to the client.


• Timesheet information– The information which appears on a consultant’s timesheet includes:

• Client company identifier and name• project id and name; client’s project manager name, address, and phone number• type of work performed (generally drawn from a list of standard activities, such as

analysis, design, programming, testing, implementation, etc.)• brief narrative description of the work performed• the amount of time, in hours and fractions, spent on each activity• an indication of whether the time should be billed to the client

• Consultant information– Information about each consultant should include their id, name, consultant

billing class, and phone number where they can be reached at the client site. Remember that a consultant may work at more than one client in any given period of time.

Entity-Relationship Diagrams

Entity-Relationship Diagram

• A model that represents system data by entity and relationship sets.

ERD Elements

• EntitiesThings about which you collect information

• RelationshipsMeans of association between entities

• IdentifiersUnique attributes of entities

• AttributesCharacteristic or property of the entity that is of interest

ERD Symbols

• Entities: rectangle• Relationships: diamond on a line with

cardinality indicated (1 to M) or lines with “chicken feet”

• Identifiers: underlined text• Attributes: text by the entity

(if shown at all on diagram)

Sample ERDs

1

1

M

1

ORDER

CAN HAVE

PART

SUPPLIER

CAN HAVE

Entity attributes:ORDER: #, DATE, PART #, QUANTITY

PART: #, DESCRIPTION, UNIT PRICE, SUPPLIER #

SUPPLIER: #, NAME, ADDRESS

Order

Part

Supplier

Entity Sets

• Each individual object is called an entity. A collection of such entities is an entity set.– Example: Joe, Jill, and Mary represent entities.

They are all ascribed to the entity set, PERSON.

– Example: A collection of projects is the project entity set.

Relationship

• Relationship– One interaction between one or more entities– For example: if a person works on a project, there is

a relationship between that person and the project• Relationship set

– A collection of such relationships.– A component in an E-R diagram that represents a

set of relationships with the same properties.

Notes on Entities and Relationships

• We can actually see entities, but we cannot see relationships.– For example:

• Entity sets: Person; project• Relationship: People work on projects. This

becomes the relationship set “Work” Persons

Projects

WorkThe set of people, set of projects and set of working relationships.

Entity-Relationship StructuresPersons

Warehouses

SuppliersParts

ProjectsDepts

Are-In Work-On Use Hold

Supply

More complex relationship:See persons and parts

Projects use partsSuppliers supply partsWarehouses hold parts

Persons are in departmentsPersons work on projects

Entities can have multiple relationships

Companies

Vehicles

Owns Leases

More than two entity sets can be associated with the same

relationship set

Customers

Parts

StoresBuy

Each relationship in this set includes a person, a part boughtby the person, and the store where the purchase was made.

Relationship sets that include only twoentities are known as binary. More thantwo are known as N-ary.

A person, a part bought by the person,and the store where the purchase was made

It is always possible to remove an N-ary relationship by replacing it

with an entity set.

Purchase

StoresCustomerParts

Of By From

Exercise

• Employees are in departments• Each department has sections.

Departments

Sections

Persons

Have

Employ

Are in

Are in is not needed

Departments

Sections

Persons

Have

Employ

Are in is not needed

Identifiers

• One of the attributes of an entity or relationship set is called the identifier

• It has one important property: its values identify unique entities in the entity set.

Identifiers are underlined here

Persons

Projects

Work

The set of people, set of projects and set of working relationships.

PERSON-IDNAMEADDRESS

PERSON-IDPROJECT-IDTIME-SPENT

PROJECT-IDSTART-DATEBUDGET

Convention for Identifiers in Relationships

• Use the identifiers of the entities that participate in the relationship as the relationship identifiers.

• Identifiers are not file keys here. At this stage, they are the identifiers of entities that participate in the relationship.

Cardinality

• The number of relationships in which one entity can appear.

• An entity can appear in:– one (1) relationship;– any variable number (N) of relationships; and– a maximum number of relationships

Cardinality - Example

Persons

Projects

Work





N

M

A persona can appear in more than oneWORK relationship, and so can a project.If there was a limit to the number of times anentity can take part in the relationship, thenN or M would be replaced by the actualmaximum number.


Manager

Projects

Manage


MANAGER-IDNAMEADDRESS



1

N

Here a project has one (1) manager, whereasa manager can manage any number (N) of projects.


Manager

Projects

Manage


MANAGER-IDNAMEADDRESS



1

N

The denotes optional participation on theproject. If it is mandatory, then there is no placed there.

Issues in building an ER-Diagram

• How to choose entities, relationships and attributes

• How to choose names• What steps should be followed

Choosing Atrributes

• Attributes, just like entity and relationship sets, should express simple concepts.

• E-R diagrams should not contain multivalued or structured attributes– For example:

PERSONSPERSON-IDDATE-OF-BIRTHQULIFICATION* (asterisk means it is multivalued-repeating)ADDRESS(NUMBER, STREET, SUBURB)Non-simple attributes

Addresses has structured attributes.These should be replaced in the final diagramby relationships.

Ex: Removing multivalued and structured attributes

Persons

ADDRESSESQUALIFICATIONS

HAVE LIVE-AT

PERSON-IDDATE-OF-BIRTH

PERSON-IDNUMBERSTREETSUBURB

NUMBERSTREETSUBURB

PERSON-IDQUALIFICATION

QUALIFICATION

Choosing Object Set Names

• Remember, that one goal of E-R modeling is to produce a model that is easily understood by users as well as computer personnel.– Entity sets are labeled as nouns– Relationship sets are labeled by verbs– Relationship sets are structured as prepositions

when modeling structural relationships (see PURCHASES example [building has rooms])

Where to begin

• Start with entity sets• Look at how entities interact with each

other and model this in terms of relationship sets

• Then add cardinality to the system• Add attributes and choose identifiers

Dependent Entities

• A dependent entity set depicts a set of entities whose existence depends on other entities.

PROJECTS

TASKS

PROJECT-IDDATE-STARTED

PROJECT-IDTASK-NOBUDGET

INVOICES

INVOICE-LINES

INVOICE-NOINVOICE-DATE

INVOICE-NOLINE-NOAMOUNT

Note: dependent entities have composite identifiers

Machines

Use

ProjectsPROJECTS

MachineAvailability

Machines

Use

N

M

N

M

SUBSETS

• Example: – generic: loan applications– Different types of loan applications which have

unique attributes

PERSONS

COURSES

TEACHERS STUDENTS

TEACHES TAKES

PERSON-IDDATE-HIRED

PERSON-IDMAJOR

PERSON-IDCOURSE-NOSEMESTER-TAUGHT

PERSON-IDCOURSE-NOSEMESTER-TAKENRESULT

COURSE-NOCOURSE-NAME

N

MM

N


Occurrence diagram for entity set PERSONSand its subsets

STAFF STUDENTS

CLUB-MEMBERS

COURTS

USE

STUDENT-IDMAJORMEMBER-NO

STAFF-IDDATE-JOINEDMEMBER-NO.

MEMBER-NO.MEMBER-GRADEDATE-JOINED-CLUB

MEMBER-NOCOURT-NONO-TIMES-USED

COURT-NOLOCATION

N

M

COMBININGSUBSETSFROM A NUMBEROF ENTITIES

entity-relationship diagrams

Documents

project record

project id

billable time record

project assignment record

time log record

activity record

time log data store3

time records