Database Lecture Notes Normalization 3 – Denormalization Dr. Meg Murray [email protected].

DatabaseLecture Notes

Normalization 3 – Denormalization

Dr. Meg Murray

[email protected]

The Normalization Question How Many Tables?

Should we store these two tables as they are, or should we combine them into one table in our new database?

Let’s Do Another Example

• What Form?

• http://www.phpbuilder.com/columns/barry20000731.php3

1NF• Eliminate repeating groups in individual tables.

• Create a separate table for each set of related data.

• Identify each set of related data with a primary key.

2NF• Create separate tables for sets of values that apply to multiple

records.• Relate these tables with a foreign key.

3NF

• Eliminate fields that do not depend on the key.

BCNF

• Are all determinant keys also candidate keys?

The Question:Normalize or Denormalize

Type of Database

• Updateable database or read-only database?

• If updateable database, we normally want tables in 3NF or BCNF

• If read-only database, we may not use normalized tables

KROENKE and AUER - DATABASE CONCEPTS (3rd Edition)© 2008 Pearson Prentice Hall

Normalization:Advantages and Disadvantages


Choosing Not to Normalize

• Normalization is used to control anomalies from functional dependencies

• There are times when normalization is not desirable

• The classic example is ZIP codes:– ZIP codes almost never change– Any anomalies are likely to be caught by normal

business practices– Not having to use two tables speeds up application

processing


Normalize ZipCode

• Is this table in 3NF?

Normalize ZipCode

Functional Dependencies:

CustomerID->(Name, Street, City, State, ZipPostalCode)

Zip->(City, State)

• Would you normalize this?

Normalize ZipCode

CUSTOMER(CustomerNumber, CustomerName, CustomerAddress, Zip, ContactName)

ZIP_CODE(Zip, City, State)

• Is this practical? KROENKE and AUER - DATABASE CONCEPTS (3rd Edition)© 2008 Pearson Prentice Hallvvvvv

Designing Read-Only Databases

Read-Only Databases

• Read-only databases are non-operational databases using data extracted from operational databases

• They are used for querying, reporting and data mining applications

• They are never updated (in the operational database sense – they may have new data imported form time-to-time)


Denormalization

• For read-only databases, normalization is seldom an advantage– Application processing speed is more

important

• Denormalization is the joining of data in normalized tables prior to storing the data

• The data is then stored in non-normalized tables


Denormalization

• Normalizing relations (or breaking them apart into many component relations) may significantly increase the complexity of the data structure

• The question is one of balance– Trading complexity for modification problems

• There are situations where denormalized relations are preferred – Google uses denormalized tables


Normalized Tables


Denormalizing the Data


Customized Tables

• Read-only databases are often designed with many copies of the same data, but with each copy customized for a specific application

• Consider the PRODUCT table:


Customized Tables

PRODUCT_PURCHASING (SKU, SKU_Description, VendorNumber, VendorName, VendorContact_1, VendorContact_2, VendorStreet, VendorCity, VendorState, VendorZip)

PRODUCT_USAGE (SKU, SKU_Description, QuantitySoldPastYear, QuantitySoldPastQuarter, QuantitySoldPastMonth)

PRODUCT_WEB (SKU, DetailPicture, ThumbnailPicture, MarketingShortDescription, MarketingLongDescription, PartColor)

PRODUCT_INVENTORY (SKU, PartNumber, SKU_Description, UnitsCode, BinNumber, ProductionKeyCode)


Database Lecture Notes Normalization 3 – Denormalization Dr. Meg Murray [email protected].

Documents

databases slide

auer database concepts

denormalize slide

pearson prentice hall

new database

denormalized tables

rd edition

separate tables