Database Management.pptx

8/14/2019 Database Management.pptx

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Third Normal Form

Problems can still existwith tables that are in

second normal form.Consider the following

Customer table:Customer ( CustomerNum, CustomerName, Balance, CreditLimit, RepNum,

LastName, FirstName)


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The functional dependenciesin this tale are as follows:

CustomerNum >

CustomerNAme, Balance,

CreditLimit, RepNum,LastName, FirstName

RepNum >LastName, FirstName


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CustomerNum determines all

the other columns.

In addition, RepNumdetermines LastName and

FirstNAme.


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When the primary key of a

table is a single column, the

table is automatically in second

normal form. Thus, the

Customer table is in second

normal form.


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Num Name Limit Num Name Name

148 AlsApplianceand Sport

$ 6550 $7500 20 Kaiser Valerie

282 BrookingsDirect

$431.20 $10000 35 Hull Richard

356 Fergusons $5785 $7500 65 Perez Juan

408 TheEverything

Shop

$5285.25

$5000.00

35 Hull Richard

524 Klines $12762 $15000 20 Kaiser Valerie

608 JohnsonsDepartment

Store

$2106 $10000 65 Perez Juan


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687 LeesSport

andAppliance

$2851 $5000 35 Hull Richard

725 DeerfieldsFour

Seasons

$248 $7500 35 Hull Richard

842 AllSeason

$8221 $7500.00

20 Kaiser Valerie

This redundancy creates the same set of problems that you

examined in the first normal form Orders table.

In addition to the problem of wasted space, you have similar

update anomalies as follows:


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1. Updates

A change to the name of a

sales rep requires not one

change to the table, but several,

making the update process

cumbersome.


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2. Inconsistent data.

There is nothing about the

design that would prohibit a salesrep from having two differentnames in the database. In fact, ifthe same sales rep represents 20customers, he or she could have

20 different names in thedatabase.


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3. Additions

In order to add sales rep 87

(Mary Daniels) to the database, shemust already represent at least onecustomer. IF she has not yet been

assigned any customers, you mustadd her record and create a fictitiouscustomer for her to represent. Again,

this is not a desirable solution to theproblem.


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4. Deletions

If you deleted all the

customers of sales rep 35 fromthe database, you would lose

all information concerning

sales rep 35.


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These update anomalies are due

to the fact that RepNum

determines LastName andFirstName, but RepNum is not the

primary key. As a result, the sameRepNum and consequently the

same LastName and FirstName

can appear on many different

rows.


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Customer Table

It is not in third normal form.The following method corrects the deficiency in the

Customer table and in all tables having similar

deficiencies.

For each determinant that is nota candidate key, remove from

the table the columns thatdepend on this determinant.


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Create a New table containing al

the columns from the original

table that depend on this

determinant.

Make the determinant the

primary key of this new table.

C t


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Customer

NumCustomerName Balanc

e

CreditLimit

RepNum

148 Als Appliance and

Sport

$6550.00 $7500.00 20

282 Brookings Direct $431.40 $10000.00 35

356 Fergusons $5785.00 $7500.00 65

408 The Everything Shop $5825.25 $5000.00 35

462 Bargains Galore $3412.00 $10000.00 65

524 Klines $12762.00 $15000.00 20

608 JohnsonsDepartment Store

$2106.00 $10000.00 65

687 Lees Sport andAppliance

$2851.00 $5000.00 35

725 Deerfields FourSeasons

$248.00 $7500.00 35


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RepNum LastName FirstName

20 Kaiser Valerie

35 Hull Richard65 Perez Juan

Conversion to third normal form

I t D iti


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Incorrect Decompositions

It is important to note that the

decomposition of a table into twoor more third normal form tablesmust be accomplished by themethod indicated in the previoussection, even though there are

other possibilities that at firstglance might seem to belegitimate.


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C t C t N l C ditLi


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Customer

NumCustomerName Balanc

e

CreditLimit

RepNum

148 Als Appliance and

Sport

$6550.00 $7500.00 20

282 Brookings Direct $431.40 $10000.00 35

356 Fergusons $5785.00 $7500.00 65

408 The Everything Shop $5825.25 $5000.00 35

462 Bargains Galore $3412.00 $10000.00 65

524 Klines $12762.00 $15000.00 20


$2106.00 $10000.00 65


$2851.00 $5000.00 35

725 Deerfields FourSeasons

$248.00 $7500.00 35

C t N L tN Fi tN


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CustomerNum LastName FirstName

148 Kaiser Valerie

282 Hull Richard356 Perez Juan

408 Hull Richard

462 Perez Juan

524 Kaiser Valerie

608 Perez Juan

687 Hull Richard

725 Hull Richard

842 Kaiser Valerie


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Consider, for example, the

redundancy in the storage of sales

reps names, the problem

encountered in changing the name

of a sales rep and the difficulty of

adding a new sales rep who

represents no customers.


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In addition, because the rep

number and names are in

different tables, you have actuallysplit a functional dependence

across two different tables. Thus,this seemingly valid

decomposition is definitely not a

desirable way to create third

normal form tables.


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There is another decomposition

that you might choose.

Customer (CustomerNum,

CustomerName, Balance,

CreditLimit, LastName,

FirstName)

CustomerN CustomerName B l CreditLim LN FN


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CustomerN

umCustomerName Balan

ce

CreditLimit

LN FN

148 Als Appliance andSport

$6550.00 $7500.00 Kaiser Valerie

282 Brookings Direct $431.40 $10000.00 Hull Richard

356 Fergusons $5785.00 $7500.00 Perez Juan

408 The Everything Shop $5825.25 $5000.00

HullRichar

d

462 Bargains Galore $3412.00 $10000.00 Perez Juan

524 Klines $12762.00 $15000.00 Kaiser Valerie


$2106.00 $10000.00 Perez Juan


$2851.00 $5000.00 Hull Richard

725 Deerfields Four

Seasons

$248.00 $7500.00

Hull

Richar

d


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Second incorrect decomposition of

the Customer Table

RepNum LastName FirstName

20 Kaiser Valerie

35 Hull Richard

65 Perez Juan


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Database Management.pptx