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Assignment One Normalising data

Normalisation is a bottom-up approach to database design that begins by examining the relationships between the attributes that in turn make up the database.

For this example of normalising data we are going to use data from AllRight Accounting which is a firm that stores data about their customers various policies with other companies.

The forms that we are extracting the data from are standard forms used by AlRight Accounting to receive the necessary information on their customers. Here the user fills in a form with details such as Personnel Details, Policy Details, and Company Details.

Aaron's Accounting

Client Number CN 4 Policy Number PN 1

Full Name Karen Hatfield Policy Description Covers holder for full Dental treatment

Monthly Debt £50 Company Number CN 100

Policy Start 1- Sep - Company Name Dave's Dental

Policy Expire 1- Sep - 00

Source of data Standard AllRight Accounting customer information form

Before we move to First Normal Form (1NF) we show how the data looks from the outset. This is called Unnormalised form (UNF).

Unnormalised Form (UNF) A table containing one or more repeating groups

customerPolicy (cliebtNo, cName, policyNo, pDesc, pStart, pExpire, mDebt, companyNo, cName)

Table Format ClientRental

clientNo cName policyNo pDesc pStart pExpire mDebt companyNo cName

CN4 Karen Hatfield PN1PN15 Covers holder for full dental treatmentCovers holder for full home insurance 1 Sep 1 Aug 1 Sep 001 Aug 00 £50£60 CN100CN110 Dave's DentalHomes Cover

CN44 Simon Hauss PN1PN0PN15 Covers holder for full dental treatmentCovers holder for full car insuranceCovers holder for full home insurance 1Oct 01 Dec 011Jan 01 1Oct 0 Dec 01Jan 0 £50£75£60 CN100CN115CN110 Dave's DentalCareful CarsHomes Cover

First Normal Form (1NF) A relation in which the intersection of each row and column contains one and only one value.

The first step of normalising the data here is to remove any repeating groups, by looking at the table in the Unnormalised form we can see that the repeating data is as follows…

Repeating Group = (policyNo, pDesc, pStart, pExpire, debt, companyNo, cName)

So to put the data in first normal form we remove the repeating group (Policy Details) by placing the repeating data along with a copy of the original key attribute (clientNo) in a separate relation. We then need to identify a primary key for the new relation (policyNo).

Client (clientNo, cName)

PolicyDetailsCompany (clientNo, policyNo, pDesc, pStart, pExpire, debt, companyNo, cName)

Table Format Client & PolicyDetailsCompany

Client

clientNo cName

CN4CN44 Karen HatfieldSimon Hauss

PolicyDetailsCompany

clientNo policyNo pDesc pStart pExpire debt companyNo cName

CN4CN4 PN1PN15 Covers holder for full dental treatmentCovers holder for full home insurance 1 Sep 1Aug 1 Sep 001 Aug 00 £50£60 CN100CN110 Dave's DentalHomes Cover

CN44CN44CN44 PN1PN0PN15 Covers holder for full dental treatmentCovers holder for full car insuranceCovers holder for full home insurance 1Oct 01 Dec 011Jan 01 1Oct 0 Dec 01Jan 0 £50£75£60 CN100CN115CN110 Dave's DentalCareful CarsHomes Cover

Second Normal Form (ND) A relation that is in first normal form and every non-primary-key attribute is fully functionally dependant on the primary key

This normal form applies to relations with composite keys (relations with a primary key composed of two or more attributes). If the relation had only a single attribute primary key it would already be in second normal form. To reach NF on our data we need to create new relations so that the non-primary key attributes are removed along with a copy of the part of the primary key that they are full functionally dependant.

Client (clientNo, cName)

Policy (clientNo, policyNo, pStart, pExpire,)

DetailsCompany (policyNo, pDesc, debt, companyNo, cName)

Table Format Client, Policy & DetailsCompany

Client

clientNo cName

CN4CN44 Karen HatfieldSimon Hauss

Policy

clientNo policyNo pStart pExpire

CN4CN4 PN1PN15 1 Sep 1Aug 1 Sep 001 Aug 00

CN44CN44CN44 PN1PN0PN15 1Oct 01 Dec 011Jan 01 1Oct 0 Dec 01Jan 0

DetailsCompany

policyNo pDesc debt companyNo cName

PN1PN15PN0 Covers holder for full dental treatmentCovers holder for full home insuranceCovers holder for full home insurance £50£60£75 CN100CN110CN115 Dave's DentalHomes CoverCareful Cars

Third Normal Form (NF) A relation that is in first and second normal form, and in which no non-primary key attribute is transitively dependent on the primary key

The normalisation from NF to NF involves the removal of any transitive dependencies. If a transitive dependency exists we should remove the transitive dependent attribute from the relation by placing the attribute in a new relation along with a copy of the determinant. In our data all the non-primary key attributes in DetailsCompany are functionally dependent on the primary key, with the exception of cName, as this is also dependent on companyNo. This is an example of a transitive dependency as the attribute cName is dependent on one or more non-primary key attributes i.e. companyNo.

So to remove this transitive dependency we create two new relations called Details and Company…

Client (clientNo, cName)

Policy (clientNo, policyNo, pStart, pExpire)

Details (policyNo, pDesc, debt, companyNo)

Company (companyNo, cName)

Table Format Client, Policy, Details & Company

Client Company

companyNo cName

CN100CN110CN115 Dave's DentalHomes CoverCareful Cars

clientNo cName

CN4CN44 Karen HatfieldSimon Hauss

Policy

clientNo policyNo pStart pExpire

CN4CN4 PN1PN15 1 Sep 1Aug 1 Sep 001 Aug 00

CN44CN44CN44 PN1PN0PN15 1Oct 01 Dec 011Jan 01 1Oct 0 Dec 01Jan 0

Details

policyNo pDesc debt companyNo

PN1PN15PN0 Covers holder for full dental treatmentCovers holder for full home insuranceCovers holder for full home insurance £50£60£75 CN100CN110CN115

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