Lec 8 Normalization 2

DatabaseLecture Notes

Normalization 2 – How to Normalize

Dr. Meg Murray

[email protected]

Normalization Why?• All relations are not equal

• Tables not normalized experience issues known as modification problems – Insertion problems

• Difficulties inserting data into a relation

– Modification problems• Difficulties modifying data into a relation

– Deletion problems• Difficulties deleting data from a relation

Deletion Anomaly

• If you delete any row, you delete information about both the machine and the repair

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

Modification Anomalies

• The EQUIPMENT_REPAIR table before and after an incorrect update operation on AcquisitionCost for Type = Drill Press:


Normalization

• Normalization is a process of analyzing a relation to ensure that it is well formed

• More specifically, if a relation is normalized (well formed), rows can be inserted, deleted, or modified without creating update anomalies


Normalization Review:Solving Modification Problems

• Most modification problems are solved by breaking an existing table into two or more tables through a process known as normalization

• So the 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?

Normal Forms

• Relations are categorized as a normal form based on which modification anomalies or other problems that they are subject to:


Normal Forms

• 1NF – A table that qualifies as a relation is in 1NF

• 2NF – A relation is in 2NF if all of its nonkey attributes are dependent on all of the primary key [focus is on composite primary keys]

• 3NF – A relation is in 3NF if it is in 2NF and has no determinants except the primary key

• Boyce-Codd Normal Form (BCNF) – A relation is in BCNF if every determinant is a candidate key


BCNF

• Boyce-Codd Normal Form (BCNF) – A relation is in BCNF if every determinant is a candidate key

“I swear to construct my tables so that all nonkey columns are dependent on the key, the whole key and nothing but the key, so help me Codd.”

Normalization Review:Definition Review

• Determinant– The attribute that can be used to find the

value of another attribute in the relation– The right-hand side of a functional

dependency

StudentID (StudentName, DormName, DormRoom)


Normalization Review:Definition Review II

• Candidate key– The value of a candidate key can be used to

find the value of every other attribute in the table

– A simple candidate key consists of only one attribute

– A composite candidate key consists of more than one attribute


The CUSTOMER Table

CUSTOMER (CustomerNumber, CustomerName, StreetAddress, City, State, ZIP, ContactName, Phone)


•What is the primary key?•What are the candidate keys?•What are the non-keyed attributes?

Simple Examples

• Remember the question:– Is every determinant a candidate key

or – are all nonkey columns dependent on the

key, the whole key and nothing but the key?

Normalization Example(StudentID) (StudentName, DormName,

DormCost)


Is every determinant a candidate key Are all nonkey columns dependent on the key, the whole key and nothing but the key?

•What are the determinants?•Does StudentID determine Student Name?•Does Student ID determine Dorm Name?•Does Student ID determine Dorm cost?

•Probably not – more likely Dorm Name does•If so, Dorm Name is a determinate of Dorm cost

•Is StudentID a candidate key?•Is Dorm Name a candidate key?

Normalization Example

(StudentID)

(StudentName, DormName, DormCost)

(DormName) (DormCost)

However, if…

(StudentID) (StudentName, DormName)

(DormName) (DormCost)

Then DormCost should be placed into its own relation,resulting in the relations:


Normalization Example

(AttorneyID,ClientID)

(ClientName, MeetingDate, Duration)

(ClientID) (ClientName)

However, if…

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

AttorneyIDClientID

ClientNameMeetingDateDuration

ATTORNEY

(ClientID) (ClientName)

(AttorneyID,ClientID)

(MeetingDate, Duration)

Then ClientName should be placed into its own relation,resulting in the relations:

AttorneyIDClientID

MeetingDateDuration

SCHEDULE

ClientID

ClientName

CLIENT

Walking through the forms

1st Normal Form [1NF]

• Eliminate Repeating Groups – Eliminate duplicative columns from the same

table. • Create separate tables for each group of related

data

– Give each table a primary key (unique identifier)

• Putting a Table into 1NF makes it a Relation– Do you remember the rules of a relation?

Is this in 1NF?

Characteristics of 1NF

• Characteristics– Table Format – No repeating groups – Primary key (PK) identified

Steps to 1NF

1. Eliminate repeating groups.– Present data in a tabular format, where each

cell has a single value and there are no repeating groups.

Repeating Groups

• Do you see the repeating group in this table?


Steps to 1NF

2. Identify the Primary Key (PK) – At a first glance the AIRCRAFT_NUMBER

seems a good candidate for a PK, but would not uniquely identify all of the remaining row attributes.

– The combination of AIRCRAFT_NUMBER and PILOT_NUMBER is a PK candidate that will uniquely identify all row attributes.

Table in 1NF

Table with primary key identified [attributes listed vertically]

Identify Dependencies• AIRCRAFT_NUMBER, PILOT_NUMBER --> AIRCRAFT_NAME,

PILOT_NAME, MISSION_CLASS, FLYING_HOUR, COST_HOUR

– Primary Key (PK) dependency. The PK is also a composite key.

• AIRCRAFT_NUMBER --> AIRCRAFT_NAME

– Partial dependency ... aircraft name is only dependent on a part of the composite AIRCRAFT_NUMBER, PILOT_NUMBER key.

Identify Dependencies

• PILOT_NUMBER --> PILOT_NAME– Partial dependency ... pilot name is only dependent on a part of the composite

AIRCRAFT_NUMBER, PILOT_NUMBER key.

• PILOT_NUMBER --> PILOT_NAME, FLYING_HOUR, COST_HOUR– Partial dependencies

• MISSION_CLASS --> COST_HOUR– Transitive dependency .... COST_HOUR non-prime/non-key attribute is

dependent on non-prime/non-key MISSION_CLASS attribute

Helpful to Create Dependency Diagram

2nd Normal Form [2NF]

• Characteristics– For Tables with composite keys– 1NF – No partial dependencies

• In other words, a non-key field must provide a fact about the whole key - not just one part of the key

2nd Normal Form [2NF]

• If an attribute depends on only part of a composite key, remove it to a separate table.– Often map to components [themes, entities…]

• Create relationships between these new tables and their predecessors through the use of foreign keys.

http://databases.about.com/od/specificproducts/a/2nf.htm

Look for Partial Dependencies

• Ask questions such as:• Are both Aircraft_Number and Pilot_Number

needed to determine Pilot_Name?• Are both Aircraft_Number and Pilot_Number

needed to determine Mission_Class?• …

Look for Partial Dependencies

• Move partial dependencies to their own tables

AIRCRAFT (Aircraft_Number)

PILOT (Pilot_Number)

FLYING HOURS (Aircraft_Number, Pilot_Number)

Steps to 2NF

Notice how moving partial dependencies separates into key components

AIRCRAFT

PILOT

FLYING HOURS

Steps to 2NF

• Assign dependent attributes to each key component – AIRCRAFT ( AIRCRAFT_NUMBER,

AIRCRAFT_NAME )

– PILOT ( PILOT_NUMBER, PILOT_NAME, MISSION_CLASS, COST_HOUR )

– FLIGHT ( AIRCRAFT_NUMBER, PILOT_NUMBER, FLYING_HOUR )

Draw New Dependency Diagram

3rd Normal Form [3NF]

• Eliminate Columns Not Dependent On Key - If attributes do not contribute to a description of the key, remove them to a separate table.– Third normal form is violated when a non-key

field is a fact about another non-key field [transitive dependency]

3rd Normal Form

• Characteristics– 2NF – No transitive dependencies

3-38

3rd Normal Form

3-39

Back to the ERD

3-40

Is the Air Pilot Example in BCNF?

• Are all determinants candidate keys?

• Often when in 3NF, also in BCNF

Steps to BCNF


Another Example

Putting a Relation into BCNF:EQUIPMENT_REPAIR


Identify Functional Dependencies

FD:

ItemNumber (Type, AcquisitionCost)

RepairNumber (ItemNumber, Type, AcquisitionCost, RepairDate, RepairAmount)

Is there a determinate key that is not a candidate key?

EQUIPMENT_REPAIR (ItemNumber, Type, AcquisitionCost,RepairNumber, RepairDate, RepairAmount)


Put into Tables

• ItemNumber is not a candidate key so– Move it and its attributes to a new table

• ITEM(ItemNumber,Type, Acquisition)

– The determinate becomes the primary key• ITEM(ItemNumber,Type, Acquisition)

– Leave a foreign key in the original table• REPAIR (ItemNumber, RepairNumber,

RepairDate, RepairAmount)


Putting a Relation into BCNF:New Relations


Putting a Relation into BCNF:SKU_DATA


Putting a Relation into BCNF:SKU_DATA

SKU_DATA (SKU, SKU_Description, Department, Buyer)

SKU (SKU_Description, Department, Buyer)

SKU_Description (SKU, Department, Buyer)

Buyer Department

SKU_DATA (SKU, SKU_Description, Buyer)

BUYER (Buyer, Department)

Where BUYER.Buyer must exist in SKU_DATA.Buyer


Putting a Relation into BCNF:New Relations


Multivalued Dependencies

• A multivalued dependency occurs when a determinant determines a particular set of values: Employee Degree

Employee Sibling

PartKit Part

• The determinant of a multivalued dependency can never be a primary key


Multivalued Dependencies


Eliminating Anomolies from Multivalued Dependencies

• Multivalued dependencies are not a problem if they are in a separate relation, so:– Always put multivalued dependencies into

their own relation– This is known as Fourth Normal Form (4NF)

References

• Example for AirPilot:– http://dotnet.org.za/willy/archive/2008/04/10/

taking-a-step-back-database-normalisation-1nf-2nf-3nf-bcnf-and-4nf-part-1.aspx

• Good reference– http://www.bkent.net/Doc/simple5.htm

Documents

Lec 8 Normalization 2