Entity-Relationship Diagrams ERD

Entity-Relationship Diagrams (ERD)

Kristian Torp

Department of Computer ScienceAalborg University

people.cs.aau.dk/˜[email protected]

November 5, 2015

daisy.aau.dk

Kristian Torp (Aalborg University) Entity-Relationship Diagrams (ERD) November 5, 2015 1 / 40

http://people.cs.aau.dk/~torp

mailto:[email protected]

http://daisy.aau.dk

Outline

1 Introduction

2 Basic StructuresEntity SetsRelationship SetsAttributesWeak Entities

3 Bad and Ugly Designs

4 ERD to Tables

5 Summary


Learning Goals

MotivationER-diagrams are widely usedER-diagrams are simple to learn

Much simpler than UML

An ER-diagram is a good communication tool

GoalsCreate non-trivial ER-diagrams

Analyze if an ER-diagram is good or bad

Create and explain the mapping of ER-diagrams to tables

Know that multiple ER-diagram notations exist


Outline

1 Introduction



4 ERD to Tables

5 Summary


ER-Diagram Example

Student

Name StudentId

Address

course

CourseId

Description

ECTS Syllabus

Takes

Grade

StartDate

StudentTakes

Name

Entity Type Relationship Type Attribute


Outline

1 Introduction



4 ERD to Tables

5 Summary


Outline

1 Introduction



4 ERD to Tables

5 Summary


Examples Entity Sets

computer

CID

Make

Model

order

OID Amount

NoteNouns in the requirement specification

ContentShould have a primary key

Should have a set of uniquely named attributes

Should have a unique name in the ER diagram

Should make sense to the customer


Entity vs. Entity Set

Entity ≈ object ≈ an instance ≈ rowEntity set ≈ class ≈ a schema ≈ table

NoteOften we just say entity when we mean entity set!


Outline

1 Introduction



4 ERD to Tables

5 Summary


Examples Relationship Sets

order order linehas

car gas stationtanking

computer motherboard cpuhas in

NoteVerbs in the requirement specification


Cardinalities

Student CourseTakes

Student CourseTakes

Student CourseTakes

Student CourseTakes

n:m

1:n

n:1

1:1


Quiz


QuestionHow many motherboards can a computer have?

many

Does the ERD support multi CPU computers? yes

Association or aggregation from UML? aggregation


QuestionHow many times can you tank your car? many times

Association or aggregation from UML? association


Quiz


QuestionHow many motherboards can a computer have? many







Quiz



Does the ERD support multi CPU computers?

yes






Quiz









Quiz




Association or aggregation from UML?

aggregation





Quiz









Quiz






QuestionHow many times can you tank your car?

many times



Quiz









Quiz







Association or aggregation from UML?

association


Quiz









Participation

Student CourseTakes

Partial ParticipationPartial participation both entities

Student CourseTakes

Full participationFull participation student

All students have to take at least one course

Partial participation courseA course does not need to have students following, e.g., a new course


Participation

Student CourseTakes

Partial ParticipationPartial participation both entities

Student CourseTakes

Full participationFull participation student

All students have to take at least one course

Partial participation courseA course does not need to have students following, e.g., a new course


Participation Quiz


QuestionCan the shop sell computers without motherboards?

yes

Can the shop sell motherboards without CPUs? no

Can the shop sell a separate CPU? no

Loan Paymentfor

QuestionDo we always know which payment a loan is for? yes


Participation Quiz


QuestionCan the shop sell computers without motherboards? yes



Loan Paymentfor



Participation Quiz



Can the shop sell motherboards without CPUs?

no


Loan Paymentfor



Participation Quiz





Loan Paymentfor



Participation Quiz




Can the shop sell a separate CPU?

no

Loan Paymentfor



Participation Quiz





Loan Paymentfor



Participation Quiz





Loan Paymentfor

QuestionDo we always know which payment a loan is for?

yes


Participation Quiz





Loan Paymentfor



Role

Self ReferenceAn entity set may be related to itself

Assume each teacher has a manager that is another teacher

teacher works for

manager

manages

NoteCan be read as (but never draw this)!

teacher teacherworks formanager manages


n-Ary Relationships

employee topic

meeting

discuss

CommentAt a number of meetings, a number of employees discuss a numberof topics

The same syntax is used in UML (classes not entity sets)


Outline

1 Introduction



4 ERD to Tables

5 Summary


Examples

student

name

student

address

street no zip

Single Composite

student

birthdate age

student

email

Derived Multi-Valued


Relationship Sets and Attributes

car

ID Make Model

gas station

ID address

tanking

DateTime

QuestionWhy the DateTime on the tanking relationship set?

The characteristics of the entity and relationship sets in therequirement specification


Outline

1 Introduction



4 ERD to Tables

5 Summary


Examples

order order linehas

QuestionAre order line depending on order?

No not in this ERD

Update DiagramMake order lines depend on an order

order

OID

order line

line no

has


Examples

order order linehas

QuestionAre order line depending on order? No not in this ERD


order

OID

order line

line no

has


Examples

order order linehas

QuestionAre order line depending on order? No not in this ERD


order

OID

order line

line no

has


Summary: Basic ConceptsConcepts

Entity setStrongWeak (no key)

Relationship setCardinality: 1-1, 1-n, n-mParticipation: Partial or fulln-ary (be careful)

AttributeSingleCompositeDerivedMulti-valued

NoteQuite simple to draw and use

Many alternative notations


Outline

1 Introduction



4 ERD to Tables

5 Summary


Bad Design, One

Student

sid cid name

Course

cid name

takes

Bad: Course ID (cid) on both entities. Fixed below!

Student

sid name

Course

cid name

takes


Bad Design, One

Student

sid cid name

Course

cid name

takes


Student

sid name

Course

cid name

takes


Bad Design, One

Student

sid cid name

Course

cid name

takes


Student

sid name

Course

cid name

takes


Bad Design, Two

Teacher

tid name room

Course

name ects

gives

Bad: No primary key on the course entity. Fixed below!

Teacher

tid name room

Course

cid name ects

gives


Bad Design, Two

Teacher

tid name room

Course

name ects

gives


Teacher

tid name room

Course

cid name ects

gives


Bad Design, Two

Teacher

tid name room

Course

name ects

gives


Teacher

tid name room

Course

cid name ects

gives


Bad Design, Three

Student

sid name

Course

cid name

takes

Bad: Not connected to the course entity. Fixed below!

Student

sid name

Course

cid name

takes


Bad Design, Three

Student

sid name

Course

cid name

takes


Student

sid name

Course

cid name

takes


Bad Design, Three

Student

sid name

Course

cid name

takes


Student

sid name

Course

cid name

takes


Bad Design, Four

Student

sid name

Course

cid name

takes

sid cid

Bad: IDs from entities on relationship. Fixed below!

Student

sid name

Course

cid name

takes


Bad Design, Four

Student

sid name

Course

cid name

takes

sid cid


Student

sid name

Course

cid name

takes


Bad Design, Four

Student

sid name

Course

cid name

takes

sid cid


Student

sid name

Course

cid name

takes


Bad Design, Five

Student

sid name

Course

cid name

takes

Good — just checking

Student

sid

Course

cid name

takes

Bad: Only key attribute on Student entity.


Bad Design, Five

Student

sid name

Course

cid name

takes


Student

sid

Course

cid name

takes



Bad Design, Five

Student

sid name

Course

cid name

takes


Student

sid

Course

cid name

takes



Bad Design, Five

Student

sid name

Course

cid name

takes


Student

sid

Course

cid name

takes



Bad Design, Six

Student

sid name

Course

cid name

takes

Bad: This does not make sense with only weak entities. Fixed below!

Student

sid name

Course

cid name

takes


Bad Design, Six

Student

sid name

Course

cid name

takes


Student

sid name

Course

cid name

takes


Bad Design, Six

Student

sid name

Course

cid name

takes


Student

sid name

Course

cid name

takes


Bad Design, Seven

Student Course

cid name

takes

Bad: No attributes on student. Fixed below!

Student

sid name

Course

cid name

takes


Bad Design, Seven

Student Course

cid name

takes


Student

sid name

Course

cid name

takes


Bad Design, Seven

Student Course

cid name

takes


Student

sid name

Course

cid name

takes


Outline

1 Introduction



4 ERD to Tables

5 Summary


Three Step Overview

1 Convert all entity sets to tables, a column for each attribute2 Convert all relationship sets to tables, a column for the primary key of

the participation entity sets3 A lot of details


Many-to-Many

A student can take many courses, a course can have many students.

Student

sid name

Course

cid name

takes

create tab l e s tudent (s id i n t e g e r pr imary key ,name varchar2 (30) not n u l l )

c reate tab l e takes (s id i n t e g e r

re ferences student ( s id ) ,c id i n t e g e r

re ferences course ( c id ) ,pr imary key ( s id , c id ) )

c reate tab l e course (c id i n t e g e r pr imary key ,name varchar2 (30) not n u l l )


One-to-Many

A student can take many courses, a course can only have one student

Student

sid name

Course

cid name

takes

create tab l e s tudent (s id i n t e g e r pr imary key ,name varchar2 (30) not n u l l )

c reate tab l e course (c id i n t e g e r pr imary key ,name varchar2 (30) not n u l l ,s i d i n t e g e r re ferences student ( s id ) )

An alternative is to use the three table approach in many-to-many


One-to-One

A student can only take one course, a course can only have onestudent

Student

sid name

Course

cid name

takes

create tab l e studentcourse (s id i n t e g e r pr imary key ,c id i n t e g e r unique not n u l l ,studentname varchar2 (30) not n u l l ,coursename varchar2 (30) not n u l l )

Ugly!

Use the two table approach listed from the one-to-many example


ERD to Tables the Hole Story

1 Regular entity types2 Weak entity types3 Binary 1:1 relationship types4 Binary 1:n relationship types5 Binary m:n relationship types6 n-ary relationship types7 Multi-valued attributes

This mapping can be automated

Is supported by various ER-modeling tools


Outline

1 Introduction



4 ERD to Tables

5 Summary


Guidelines

Good AdviseUse the same ERD notation through out the project

Each entity set should have a primary key

Be careful not to repeat information

Use composite attributes where appropriate

Watch out for n-ary relationships (they bite!)

Watch of for cardinalities and participation constraintsUse a tool, that can map ERDs to tables!

An tables to ERD

Stress TestPrint ERD on single page (no attributes)

Explain the diagram to your neighbor!


Summary

Main PointsWidely used

Unfortunately there is not an ERD standard

Extended ER modelingSupports inheritanceSupports categories

Many ER modeling tools

Alternatives to ERDs are UML or ODL


Data & Analytics

Entity-Relationship Diagrams ERD