Chapter 5 DBMS

8/9/2019 Chapter 5 DBMS

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Database Systems, 8th Edition 2

Objectives

• In this chapter, you will learn:

– What normalization is and what role it plays inthe database design process

– About the normal orms !"#, $"#, %"#, &'"#,and ("#

– )ow normal orms can be transormed romlower normal orms to higher normal orms

– *hat normalization and + modeling are usedconcurrently to produce a good database design

– *hat some situations re-uire denormalization togenerate inormation eiciently


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.atabase *ables and "ormalization

• Normalization

– /rocess or evaluating and correcting table

structures to minimize data redundancies

• educes data anomalies

– Wor0s through a series o stages called normal

orms:

• #irst normal orm 1!"#2

• 3econd normal orm 1$"#2

• *hird normal orm 1%"#2


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.atabase *ables and "ormalization

1continued2

• "ormalization 1continued2

– $"# is better than !"#4 %"# is better than $"#

– #or most business database design purposes,

%"# is as high as needed in normalization

– )ighest level o normalization is not always most

desirable

• Denormalization produces a lower normal orm – /rice paid or increased perormance is greater

data redundancy


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*he "eed or "ormalization

• +5ample: company that manages building

projects

– 'harges its clients by billing hours spent on

each contract

– )ourly billing rate is dependent on employee6s

position

– /eriodically, report is generated that contains

inormation such as displayed in *able 78!


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*he "eed or "ormalization

1continued2

• 3tructure o data set in #igure 78! does not

handle data very well

• *able structure appears to wor04 report

generated with ease

• eport may yield dierent results depending on

what data anomaly has occurred

• elational database environment suited to helpdesigner avoid data integrity problems


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*he "ormalization /rocess

• +ach table represents a single subject

• "o data item will be unnecessarily stored in

more than one table

• All attributes in a table are dependent on the

primary 0ey

• +ach table void o insertion, update, deletion

anomalies


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*he "ormalization /rocess

1continued2

• Objective o normalization is to ensure all tables

in at least %"#

• )igher orms not li0ely to be encountered in

business environment

• "ormalization wor0s one relation at a time

• /rogressively brea0s table into new set o

relations based on identiied dependencies


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'onversion to #irst "ormal #orm

• epeating group

– 9roup o multiple entries o same type e5ist or

any single 0ey attribute occurrence

• elational table must not contain repeatinggroups

• "ormalizing table structure will reduce data

redundancies• "ormalization is threestep procedure


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1continued2

• 3tep !: +liminate the epeating 9roups

– +liminate nulls: each repeating group attribute

contains an appropriate data value

• 3tep $: Identiy the /rimary ;ey

–


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1continued2

• .ependency diagram:

– .epicts all dependencies ound within given

table structure

– )elpul in getting bird6seye view o allrelationships among table6s attributes

–


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1continued2

• #irst normal orm describes tabular ormat inwhich:

– All 0ey attributes are deined

– *here are no repeating groups in the table

– All attributes are dependent on primary 0ey

• All relational tables satisy !"# re-uirements

• 3ome tables contain partial dependencies – .ependencies based on part o the primary 0ey

– 3hould be used with caution


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'onversion to 3econd "ormal #orm

• 3tep !: Write +ach ;ey 'omponenton a 3eparate =ine

– Write each 0ey component on separate line, then

write original 1composite2 0ey on last line – +ach component will become 0ey in new table

• 3tep $: Assign 'orresponding .ependent Attributes

– .etermine those attributes that are dependenton other attributes

– At this point, most anomalies have beeneliminated


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'onversion to 3econd "ormal #orm

1continued2

• *able is in second normal orm 1$"#2 when:

– It is in !"# and

– It includes no partial dependencies:

• "o attribute is dependent on only portion o

primary 0ey


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'onversion to *hird "ormal #orm

• 3tep !: Identiy +ach "ew .eterminant

– #or every transitive dependency, write its

determinant as /; or new table

– .eterminant: any attribute whose valuedetermines other values within a row

• 3tep $: Identiy the .ependent Attributes

– Identiy attributes dependent on eachdeterminant identiied in 3tep !

• Identiy dependency

– "ame table to relect its contents and unction


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1continued2

• 3tep %: emove the .ependent Attributes rom

*ransitive .ependencies

– +liminate all dependent attributes in transitive

relationship1s2 rom each o the tables – .raw new dependency diagram to show all

tables deined in 3teps !–%

– 'hec0 new tables as well as tables modiied in

3tep %

• +ach table has determinant

• "o table contains inappropriate dependencies


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1continued2

• A table is in third normal orm 1%"#2 when both

o the ollowing are true:

– It is in $"#

– It contains no transitive dependencies


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Improving the .esign

• *able structures cleaned up to eliminate initial

partial and transitive dependencies

• "ormalization cannot, by itsel, be relied on to

ma0e good designs

• It is valuable because its use helps eliminate

data redundancies


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Improving the .esign 1continued2

• Issues to address in order to produce a goodnormalized set o tables: – +valuate /; Assignments

– +valuate "aming 'onventions – eine Attribute Atomicity – Identiy "ew Attributes – Identiy "ew elationships

– eine /rimary ;eys as e-uired or .ata9ranularity – sing .erived Attributes


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3urrogate ;ey 'onsiderations

• When primary 0ey is considered to be

unsuitable, designers use surrogate 0eys

• .ata entries in *able 78% are inappropriate

because they duplicate e5isting records – "o violation o entity or reerential integrity


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)igher=evel "ormal #orms

• *ables in %"# perorm suitably in business

transactional databases

• )igher order normal orms useul on occasion

• *wo special cases o %"#:

– &oyce'odd normal orm 1&'"#2

– #ourth normal orm 1("#2


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*he &oyce'odd "ormal #orm

1&'"#2

• +very determinant in table is a candidate 0ey

– )as same characteristics as primary 0ey, but or

some reason, not chosen to be primary 0ey

• When table contains only one candidate 0ey,the %"# and the &'"# are e-uivalent

• &'"# can be violated only when table contains

more than one candidate 0ey


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*he &oyce'odd "ormal #orm

1&'"#2 1continued2

•


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Database Systems, 8th

Edition 33


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Edition 34


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Edition 35


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Edition 37


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Edition 38


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Edition 39

"ormalization and .atabase .esign

• "ormalization should be part o the design

process

•


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Edition 40

"ormalization and .atabase .esign

1continued2

• + diagram

– Identiy relevant entities, their attributes, and

their relationships

– Identiy additional entities and attributes

• "ormalization procedures

– #ocus on characteristics o speciic entities

–


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Edition 41


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Edition 42


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Edition 43


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Edition44


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Edition45


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Edition46

.enormalization

• 'reation o normalized relations is important

database design goal

• /rocessing re-uirements should also be a goal

• I tables decomposed to conorm tonormalization re-uirements:

– "umber o database tables e5pands


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Edition47

.enormalization 1continued2

• @oining the larger number o tables reducessystem speed

• 'onlicts oten resolved through compromises

that may include denormalization• .eects o unnormalized tables:

– .ata updates are less eicient because tablesare larger

– Inde5ing is more cumbersome – "o simple strategies or creating virtual tables

0nown as views


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Edition48

3ummary

• "ormalization is used to minimize data

redundancies

• #irst three normal orms 1!"#, $"#, and %"#2

are most commonly encountered• *able is in !"# when:

– All 0ey attributes are deined

– All remaining attributes are dependent onprimary 0ey


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Edition49

3ummary 1continued2

• *able is in $"# when it is in !"# and contains no

partial dependencies

• *able is in %"# when it is in $"# and contains no

transitive dependencies• *able that is not in %"# may be split into new tables

until all o the tables meet %"# re-uirements

• "ormalization is important partbut only parto

the design process


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3ummary 1continued2

• *able in %"# may contain multivalueddependencies

– "umerous null values or redundant data

• 'onvert %"# table to ("# by: – 3plitting table to remove multivalued

dependencies

• *ables are sometimes denormalized to yield

less IBO, which increases processing speed

Documents

Chapter 5 DBMS