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Chapter 2. The Relational Model

Chapter 1 Review

• Potential problems with Lists– Deletion– Update– Insertion

• Avoid these problems using a relational database:– Break a list into several tables, one table for each theme– Tables can be joined together using the value of the data

• So far, we are pretty vague on what we mean by “table”, “theme” and “value of the data”……

Student Course

Chapter 2 Objective

• Learn the concept of the relational model

• Learn the meaning and importance of keys, foreign keys, and related terminology

• Learn the meaning of functional dependencies

• Learn to apply a process for normalizing relations

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Entity

• An entity is something of importance to a user that needs to be represented in a database

• An entity represents one theme or topic– An object (e.g., book) with various characteristics

(e.g., authors, publisher, ISSN)

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Relation

• A relation is a two-dimensional table that has specific characteristics

• The table dimensions, like a matrix, consist of rows and columns

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Characteristics of a Relation• Columns contain data about attributes of the entity

class– Each column has a unique name– All entries in a column are the same kind

• Rows contain data about an entity instance– Cells of the table hold a single value– No two rows may be identical

• The order of the columns is unimportant• The order of the rows is unimportantStudentID FirstName LastName DOB

9123450 John Smith Jan. 1, 1989

9123451 John Adam Jun. 1, 1988

9123452 Jane Adam Aug, 1,1989

9123453 Josh Cohen Aug. 1,1989

Exercise: Relation or Non-Relation?

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EmployeeNumber Phone LastName

100 335-6421,454-9744

Abernathy

101 215-7789 Cadley

104 610-9850 Copley

107 299-9090 Jackson

Cells of the table hold multiple values

Exercise: Relation or Non-Relation?

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EmployeeNumber Phone LastName

100 335-6421 Abernathy

101 215-7789 Cadley

104 610-9850 Copley

100 335-6421 Abernathy

107 299-9090 Jackson

No two rows may be identical

Exercise: Relation or Non-Relation?

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EmployeeNumber FirstName LastName100 Mary Abernathy101 Jerry Cadley104 Alex Copley107 Megan Jackson

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Presenting Relation Structure

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Column 1 Column 2 … Column n RELATION_NAME(Column1, Column 2, …, Column n)

RELATION_NAME

STUDENT(StudentID, FirstName, LastName, DOB)

StudentID FirstName LastName DOB

9123450 John Smith Jan. 1, 1989

9123451 John Adam Jun. 1, 1988

9123452 Jane Adam Aug, 1,1989

9123453 Josh Cohen Aug. 1,1989

STUDENT

Original Table Relation Representation

From now on, we will frequently use this presentation for relations

Terminology

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Synonyms…

Table Row Column

File Record Field

Relation Tuple Attribute

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Key

• A key is one (or more) columns of a relation that is (are) used to uniquely identify a row– E.g.: OrderID is a key to uniquely identify an order– A composite key is a key that contains two or more

attributes• E.g.: (BuildingNumber, ApartmentNumber) is a composite

key to uniquely identify an apartment

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Example: KeyStudentID FirstName LastName DOB9123450 John Smith Jan. 1, 19899123451 John Adam Jun. 1, 19889123452 Jane Adam Aug, 1,19899123453 Josh Cohen Aug. 1,1989

What attribute(s) form a key?

StudentID: FirstName:(FirstName, LastName):

(FirstName, LastName, DOB): (StudentID, FirstName): (StudentID, FirstName, LastName, DOB):

yesnoyes (in this table), but no (if there are thousands of records, there could be students with same first name and last name)yes (in this table), but no (if more records)yes, but …yes, but …

A key is one (or more) columns of a relation that is (are) used to uniquely identify a row.

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Candidate Key

What attribute(s) form a key?

StudentID: FirstName:(FirstName, LastName):

(FirstName, LastName, DOB): (StudentID, FirstName): (StudentID, FirstName, LastName, DOB):

yesnoyes (in this table), but no (if there are thousands of records, there could be students with same first name and last name)yes (in this table), but no (if more records)yes, butyes, but

StudentID FirstName LastName DOB

9123450 John Smith Jan. 1, 1989

9123451 John Adam Jun. 1, 1988

9123452 Jane Adam Aug, 1,1989

9123453 Josh Cohen Aug. 1,1989

• A candidate key is called “candidate” because it is a candidate to become the primary key– A special key– If the subset of a key is also a key, we usually don’t

consider it as a candidate key

not a candidate keynot a candidate key

, a candidate key

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Primary Key• A primary key is a candidate key chosen to be the main key

for the relation– A relation can only have one primary key– Each candidate key could be chosen as a primary key, but we usually

have preferencesStudentID FirstName LastName DOB

9123450 John Smith Jan. 1, 1989

9123451 John Adam Jun. 1, 1988

9123452 Jane Adam Aug, 1,1989

9123453 Josh Cohen Aug. 1,1989

Primary key: StudentID

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Primary Key: Discussion

Even if HomeAddress could be a candidate key, we still prefer choosing StudentID as the primary key. Because (1) HomeAddress might have duplicate(2) HomeAddress is a string, hard to index and query. StudentID is numeric

value

STUDENT(StudentID, FirstName, LastName, DOB, SSN)

SSN: Candidate key? Good to be a primary key?

STUDENT(StudentID, FirstName, LastName, DOB, HomeAddress)

HomeAddress: Candidate key? Good to be a primary key?

Even if SSN is a candidate key, we still prefer choosing StudentID as the primary key. Because SSN is sensitive information

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Presenting Primary Key

• Single KeyRELATION_NAME(Column1, Column 2, …, Column n)

• STUDENT(StudentID, FirstName, LastName, DOB)– The underline of StudentID indicates StudentID is the primary

key of this relation

• Composite KeyRELATION_NAME(Column1, Column 2, …, Column n)

• REGISTRATION(StudentID, CourseID, RegistrationDate)– The underline of StudentID and CourseID indicates (StudentID,

CourseID) is the composite primary key of this relation

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How to Choose a Primary Key?CustomerName HomeAddress DOB

John Smith 293 Main St Jan. 1, 1989John Adam 10 Green Rd Jun. 1, 1988Jane White 111 University Aug, 1,1989Josh Cohen 12 Beaver Aug. 1,1989

What if none of existing attributes is appropriate?Answer: artificially create a new attribute

Candidate keys: CustomerName? HomeAddress? DOB?Primary key?

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A Surrogate Key

• A Surrogate Key is a unique numeric value that is added to a relation to serve as the primary key– System generated– Contains no semantic meaning

• Surrogate key is very commonly used. A surrogate key is often used to replace a composite primary key or a non-numeric primary key– (FirstName, LastName, DOB) StudentID– HomeAddress CustomerID

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Surrogate Key Examples

• Penn State database– StudentID

• Membership database– MembershipID

• Online shopping– OrderNumber

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Review

• Key: StudentID, (StudentID, FirstName), …• Candidate key: StudentID• Primary key: (StudentID, FirstName, LastName, DOB)• Surrogate key: StudentID

StudentID FirstName LastName DOB9123450 John Smith Jan. 1, 19899123451 John Adam Jun. 1, 19889123452 Jane Adam Aug, 1,19899123453 Josh Cohen Aug. 1,1989

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Exercise: True/False

• Candidate keys may or may not be unique.

• The primary key is used to identify unique rows in a relation.

• Surrogate key values have no meaning to the users.

• Surrogate key is not primary key.

Exercise: True/False

• Suppose we have the following table:BOOK(BookID, Title, Publisher, Year)

• Q1. Title is a key in BOOK?• Q2. (BookID, Title) is a key?• Q3. (BookID, Title) is a candidate key?• Q4. BookID is a surrogate key added to serve

as a primary key?

And one more question…

• What’s the primary key of this table?

EmployeeID EmployeeName Skill Work Location

101 Jones Typing 114 Main Street

101 Jones Shorthand 114 Main Street

101 Jones Whittling 114 Main Street

102 Bravo Light Cleaning 73 Industrial Way

103 Ellis Alchemy 73 Industrial Way

103 Ellis Flying 73 Industrial Way

104 Harrison Light Cleaning 73 Industrial Way