copyright © 1998-2011 curt hill entities and relationships the basics and what they have to do with...

Copyright © 1998-2011 Curt Hill

Entities and Relationships

The basics and what they have to do with database


Entities and Relationships• Entity

– Usually a real world thing• College: faculty, course, students

– Each entity has attributes• People have names, ages• Courses have names, hours, times

• Relationship– A logical connection between two or

more entities– These may have attributes


Relationships

• In mathematics a relation is an represented by an ordered n-tuple

• Each record is an ordered n-tuple– Every table expresses relationship– Table is a file with just one record

type

• Relationships may span tables


Relationship Arity or Cardinality

• How many are on either side of the relationship

• One to one– A person and SSN– Keys need this arity

• One to many– Instructor to student in a course

• Many to many– People to organizations– Hardest to represent


Table Use• We use a table to collect a group

of entities of the same type– The student table

• The attributes of the relationship form the fields of a record

• One to one relationships are just a field in the table

• One to many relationships and many to many relationships are often their own table


ER Model• The Entity-Relationship model

helps us to think about and design a database

• The only data in a database consists of entities, relationships and their attributes

• Helps us to bridge the gap between informal user desires to a formal database design


Entity-Relationship Diagrams• A pictorial mechanism to show

what is going on• Not standardized

– They come in many variations

• Entities – rectangles• Attributes – ovals• Relationships – diamonds• Connections – lines


A simplified ER diagram

Licenses

Manufacturer

Dealer

Car

Shipper

Builds

Stocks

Contracts

Transports


A manufacturing ER diagram

• Last one had no attributes for several reasons

• Often that is how we get started– Refine as we go along

• These tend to be both formal and informal

• The graphic had no space


ER diagram Again

Licenses

Manufacturer

Dealer

Name Address

TypeStartDate

Address

Fee

Name

MID

Lines

DID

LID


Legend on the above• Two perpendicular lines indicate an

arity of one• Terminating with three lines indicates

an arity of many• Underlined name indicates key

– There may be more than one

• Relationships do not have to be between just two different types of entities– Binary, ternary, n-ary– One or more different tables

Variants

• The above diagram is often called Crow’s Foot diagram– The arity relationship is done using

crow’s feet or perpendicular lines

• The alternative is called Chen notation– This must 1,n,m to identify



ER diagram Again

Licenses

Manufacturer

Dealer

Name Address

TypeStartDate

Address

Fee

Name

MID

Lines

DID

LID

1

M


Participation Constraints

• Total: every record in the table participates in this relationship– All manufacturers have dealers– Usually a heavy line

• Partial: there may be some records without this relationship– Suppose dealers includes used car

dealers– Some have no license from a

manufacturer– Denoted by a light line


Some Definitions

• Entity set– Any set of similar entities– Could be a whole table or subset of

this table’s records

• Relationship set


Steps for DB Design

• Requirements• Conceptual• Logical• Schema Refinement• Physical Design• Application• Security


Requirements

• What to find– What data is needed– What applications will be needed

• How to go about it– Interview the staff– Study any prior systems


Conceptual

• When the requirements are understood, then take a shot at the high level design

• This is a high level approach• A simple model is required since it

must be understood by non-technical personnel


Logical

• Choose a DBMS• Translate conceptual design into a

schema supportable by the DBMS– Table definitions– Relationships– Primary and foreign keys


Schema Refinement

• It is never perfect the first time• Put the tables in the desired

normal form• There are objective techniques to

drive the process


Physical Design

• Performance is the key in this step• What expected usage will be• Where to use indexing or other

performance enhancements or re-factor the tables


Applications

• The requirements of the applications that interface with the database cause the last refinement

• These applications may interface with the web as well


Security

• Tailoring the views with security in mind

copyright © 1998-2011 curt hill entities and relationships the basics and what they have to do with...

Documents

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