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§2 – The Relational Model and Normalization (cont.)

Fen Wang

wangfen@cwu.edu

Lecture 6

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Agenda

• The foundation of the relational model

• Characteristics of relations

• The basic relational terminology

• Primary, candidate, and composite keys

• The purpose and use of surrogate keys

• How foreign keys represent relationships

• The meaning of functional dependencies

• Possible insertion, deletion, and update anomalies

• Apply a process for normalizing relations

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Functional Dependency

• A functional dependency occurs when one attribute (or group of attributes) determines the value of another attribute in the same table

StudentID StudentName

StudentID (DormName, DormRoom, Fee)

• The attribute on the left side of the functional dependency is called the determinant

• Functional dependencies may be based on equations:

ExtendedPrice = Quantity X UnitPrice

(Quantity, UnitPrice) ExtendedPrice

• Function dependencies are not equations!

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Functional Dependencies That Are Not Equations

ObjectColor Weight

ObjectColor Shape

ObjectColor (Weight, Shape)

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Candidate/Primary Keys and Functional Dependency

• By definition… A candidate key of a relation will functionally determine all other attributes in the row

• Likewise, by definition… A primary key of a relation will functionally determine all other attributes in the row

(EmployeeID)

(EmpLastName, EmpPhone)

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Functional Dependencies in the REP Table

Does any column determine the value of another?

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Functional Dependencies in the REP Table (cont.)

RepNum (LastName, FirstName, Address, PayClass, Rate)

(LastName, FirstName) (RepNum, Address, PayClass, Rate)

PayClass Rate

Instances in a relation can NOT prove that a functional dependency exists; however, they can be used to

demonstrate that a functional dependency does NOT exist.

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Functional Dependencies in the REP Table (cont.)

With the new dataset given, the 2nd possible functional dependency can now be rejected:

(LastName, FirstName) (RepNum, Address, PayClass, Rate)

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Functional Dependencies in the ORDER_ITEM Table

What are the functional dependencies in this table?

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Functional Dependencies in the ORDER_ITEM Table

(OrderNumber, SKU)

(Quantity, Price, ExtendedPrice)

(Quantity, Price) (ExtendedPrice)

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Functional Dependencies in the Campus Building Scenario

• Which of the following functional dependency diagrams accurately represents the following situation:

– A campus has many buildings.

– Each building has a unique name.

– Each building has many rooms.

– All rooms in any given building are numbered sequentially starting at "101".

– Each room has a certain capacity although many rooms in the same building or different buildings may have the same capacity.

– Each room is assigned to a single department.

– A department may have many rooms in one or more buildings, each with the same or different capacities.

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Functional Dependencies in the Campus Building Scenario (cont.)

• Which of the following functional dependency diagrams accurately represents the situation (described in the preceding slide):

A) BuildingName → (RoomNumber, Capacity, Department)

B) RoomNumber → (BuildingName, Department, Capacity)

C) (Department, Capacity) → (BuildingName, RoomNumber)

D) (BuildingName, RoomNumber) → (Capacity, Department)

Sample answer:

D

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Agenda

• The foundation of the relational model

• Characteristics of relations

• The basic relational terminology

• Primary, candidate, and composite keys

• The purpose and use of surrogate keys

• How foreign keys represent relationships

• The meaning of functional dependencies

• Possible insertion, deletion, and update anomalies

• Apply a process for normalizing relations

Modification Problems (cont.)

• What are the modification problems in this PROJECT_EMPLOYEE table?

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Modification Problems (cont.)

• What are the modification problems in this PROJECT_EMPLOYEE table?

– Update: Changes to employee data may need to be made in several rows.

– Delete: Deleting data for a project or employee deletes project hours data as well.

– Insert: Adding a new project record with missing data on employee or project hours

The Source of Modification Problems

• Why do these problems exist?

– When a table contains data from multiple themes, it is subject to modification problems

• This results in data duplication and an unnecessary dependency between the entities

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Normalization

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

– To be a well-formed relation, every determinant must be a candidate key

– Any relation that is not well formed should be broken into two or more well-formed relations

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Normal Forms

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

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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

• 3NF – A relation is in 3NF if it is in 2NF and has no functional dependencies between non-key attributes

• 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.”

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Normal Forms (cont.)

• Normalization

– 2NF is better than 1NF; 3NF is better than 2NF, and BCNF is better than 3NF

– For most business database design purposes, BCNF is as high as we need to go in normalization process

– Highest level of normalization is not always most desirable

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The Normalization Process: An Example

• Example: A customer invoice from Pine Valley Furniture Company

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The Normalization Process: An Example (cont.)

• The table representing the invoice

Table with repeating groups, not in 1st normal form

Note: this is NOT a relation

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The Normalization Process: An Example (cont.)

Conversion to First Normal Form

• A table is in 1NF if the following two constraints both apply:

– There’re no repeating groups in the table

– A primary key can be defined to uniquely identify each row in the table

Smart tip: if a table qualifies to be a relation, it is in 1NF. In other words,

a relation is a 1NF table!

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Table with no repeating groups and unique rows, in 1NF

Note: this is a relation, but not a well-structured one

The Normalization Process: An Example (cont.)

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The Normalization Process: An Example (cont.)

Conversion to First Normal Form (Summary)

• A table is in 1NF when there’re no repeating groups in the table and a primary key can be defined to uniquely identify each row in the table

• Some may contain partial dependencies

– Dependencies based on only part of the primary key

– Still subject to data redundancies

INVOICE (Order_ID, Order_date, Customer_ID, Customer_Name,

Customer_Address,Product_ID, Product_Description,

Product_Finish, Unit_Price, Ordered_Quantity)

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The Normalization Process: An Example (cont.)

Conversion to Second Normal Form

• Table is in second normal form (2NF) when:

– It is in 1NF and

– It includes no partial dependencies:

• No attribute is dependent on only portion of primary key

• called full functional dependency

– At this point, many anomalies have been eliminated

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Order_ID Order_Date, Customer_ID, Customer_Name, Customer_Address

Therefore, NOT in 2nd Normal Form

Product_ID Product_Description, Product_Finish, Unit_Price

Functional dependency diagram for INVOICE

The Normalization Process: An Example (cont.)

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The Normalization Process: An Example (cont.)

Conversion to Second Normal Form

1. Identify all partial dependencies

2. Place the columns of the functional dependency in a new table of their own

3. Make the determinant the primary key of the new table

4. Leave a copy of the determinant as a foreign key in the original table

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Partial dependencies are removed, but there

are still transitive dependencies

Getting it into Second Normal Form

Removing partial dependencies

The Normalization Process: An Example (cont.)

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The Normalization Process: An Example (cont.)

Conversion to Second Normal Form (Summary)

• 2nd Normal Form: If each nonkey attribute is functionally dependent on the whole primary key

• No attribute is dependent on only portion of primary key

• called full functional dependency

• Some tables still contain transitive dependencies

– Dependencies among non-key attributes

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The Normalization Process: An Example (cont.)

Conversion to Third Normal Form

• A table is in third normal form (3NF) when both of the following are true:

– It is in 2NF

– It contains no transitive dependencies

• i.e., no functional (transitive) dependencies between two (or more) non-key attributes

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The Normalization Process: An Example (cont.)

Conversion to Third Normal Form

1. Identify all transitive dependencies

2. Place the columns of the functional dependency in a new table of their own

3. Make the non-key determinant the primary key of the new table

4. Leave a copy of the non-key determinant as a foreign key in the original table

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Transitive dependencies are removed

Removing transitive dependencies

Getting it into Third Normal Form

The Normalization Process: An Example (cont.)

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The Normalization Process: An Example (cont.)

• The 3NF tables representing the original customer invoice

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The Boyce-Codd Normal Form (BCNF)

• Every determinant in table is a candidate key

– A candidate key of a relation will functionally determine all other attributes in the row

• When table contains only one candidate key, the 3NF and the BCNF are equivalent

• Most designers consider the BCNF as a special case of 3NF

• Any modification problem arising from functional dependencies are removed

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Before next class

• Practice more with MS Access 2010 and MS Visio Professional

• Continue working on Project D1

• Continue working on Assignment#2 (due 10PM on Friday)

• Read Textbook Chapter 2 – Functional Dependencies