Normalization in DBMS: 1NF, 2NF, 3NF and BCNF in Database 07... · 2021. 2. 15. · instances of an entity. It acts as a cross-reference between two tables as it references the primary

DATABASE II

Computer science department

Second stage

Second semester

2020-2021

Asst.lecturer Wafaa Mustafa

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DBMS KEYS

• KEYS in DBMS is an attribute or set of attributes which

helps you to identify a row(tuple) in a relation(table).

• They allow you to find the relation between two tables.

• Keys help you uniquely identify a row in a table by

a combination of one or more columns in that table.

• Key is also helpful for finding unique record or row from the

table

• Database key is also helpful for finding unique record or row

from the table.

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Types of Keys in Database Management System

• Super Key - A super key is a group of single or multiple keys

which identifies rows in a table.

• Primary Key - is a column or group of columns in a table that uniquely identify every row in that table.

• Candidate Key - is a set of attributes that uniquely identify tuples in a table. Candidate Key is a super key with no repeated attributes.

• Foreign Key - is a column that creates a relationship between two tables. The purpose of Foreign keys is to maintain data integrity and allow navigation between two different instances of an entity.

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Emp-SSN Emp-Num Emp-name

9812345098 AB05 Shown

9876512345 AB06 Roslyn

199937890 AB07 James

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SUPERKEY is a group of single or multiple keys which identifies

rows in a table.

A Super key may have additional attributes that are not needed

for unique identification.

Example:

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PRIMARY KEY is a column or group of columns in a table

that uniquely identify every row in that table. The Primary Key

can't be a duplicate meaning the same value can't appear more

than once in the table. A table cannot have more than one

primary key.

Rules for defining Primary key:

•Two rows can't have the same primary key value

•It must for every row to have a primary key value.

•The primary key field cannot be null.

•The value in a primary key column can never be modified or

updated if any foreign key refers to that primary key.

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Example: In the following table,( StudID) is a Primary Key.

• CANDIDATE KEY is a set of attributes that uniquely identify tuples in a table. Candidate Key is a super key with no repeated attributes. The Primary key should be selected from the candidate keys.

• Every table must have at least a single candidate key.

• A table can have multiple candidate keys but only a single primary key.

• Properties of Candidate key:

• It must contain unique values;

• Candidate key may have multiple attributes;

• Must not contain null values;

• It should contain minimum fields to ensure uniqueness;

• Uniquely identify each record in a table.

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.

• Example: In the given table Stud ID, Roll No, and email are

candidate keys which help us to uniquely identify the student

record in the table

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StudID Roll No First Name LastName Email

1 11 Tom Price [email protected]

2 12 Nick Wright [email protected]

3 13 Dana Natan [email protected]

candidate key

primary key

mailto:[email protected]



• FOREIGN KEY is a column that creates a relationship

between two tables. The purpose of Foreign keys is to maintain

data integrity and allow navigation between two different

instances of an entity. It acts as a cross-reference between two

tables as it references the primary key of another table.

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DeptCode DeptName

001 Science

002 English

005 Computer

Example:

Teacher

ID

Fname Lname

B002 David Warner

B017 Sara Joseph

B009 Mike Brunton

• In this key in dbms example, we have two table, teach and

department in a school. However, there is no way to see which

search work in which department.

• In this table, adding the foreign key in Deptcode to the Teacher

name, we can create a relationship between the two tables.

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Teacher ID DeptCode Fname Lname

B002 002 David Warner

B017 002 Sara Joseph

B009 001 Mike Brunton

This concept is also known as Referential Integrity.

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RELATIONSHIP

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Relationship

• Relationship The association among entities is called a

relationship. For example, an employee works at a

department, a student enrolls in a course. Here, Works at

and Enrolls are called relationships.

Asst lecturer .Wafaa Mustafa 14

One-to-one:

• In a one-to-one relationship, one record in a table is

associated with one and only one record in another table.

For example, in a school database, each student has

only one student ID, and each student ID is assigned to

only one person.


Types of Relationship

Types of Relationship cont.


One-to-many:

O In a one-to-many relationship, one record in a table

can be associated with one or more records in another

table. For example, each customer can have many sales

orders.



Many-to-one:

O More than one entities from entity set A can be

associated with at most one entity of entity set B,

however an entity from entity set B can be associated

with more than one entity from entity set A.

A University database let us consider two entity sets

Student and Course. Assume that a student can

register many courses that are offered and a course may be

registered by many students per semester. Then

the relationship between Student and Course will be

many-to-many.



O Many-to-many: One entity from A can be associated

with more than one entity from B and vice versa.

A typical example of a many-to many relationship is

one between students and classes. A student can register

for many classes, and a class can include many students.


One customer may

place many orders,

but each order is

placed by a single

customer

One-to-many

relationship


One order has many order

lines; each order line is

associated with a single

order

One-to-many relationship


One product can be in

many order lines, each

order line refers to a

single product

One-to-many

relationship


Product Supplier

Product

Stock Warehouse Purchase

Order

Purchase

Order Item

Co

nta

ins

Sto

cked

as

Ordered on Supplies

Holds


Relationships established in special columns that provide links between

tables

NORMALIZATION IN

DBMS: 1NF, 2NF, 3NF

Normalization

• is a process of organizing the data in database to avoid

data redundancy, insertion anomaly, update anomaly &

deletion anomaly. Let’s discuss about anomalies first then

we will discuss normal forms with examples

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• There are three types of anomalies that occur when the

database is not normalized. These are – Insertion, update

and deletion anomaly. Let’s take an example to

understand this.

• Example: Suppose a manufacturing company stores the

employee details in a table named employee that has four

attributes: emp_id for storing employee’s id, emp_name

for storing employee’s name, emp_address for storing

employee’s address and emp_dept for storing the

department details in which the employee works. At some

point of time the table looks like this:

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emp_id emp_name emp_address emp_dept

101 Rick Delhi D001

101 Rick Delhi D002

123 Maggie Agra D890

166 Glenn Chennai D900

166 Glenn Chennai D004

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The above table is not normalized. We will see the

problems that we face when a table is not normalized:

Update anomaly: In the above table we have two rows

for employee Rick as he belongs to two departments of

the company. If we want to update the address of Rick

then we have to update the same in two rows or the data

will become inconsistent. If somehow, the correct address

gets updated in one department but not in other then as

per the database, Rick would be having two different

addresses, which is not correct and would lead to

inconsistent data.

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• Insert anomaly: Suppose a new employee joins the

company, who is under training and currently not assigned

to any department then we would not be able to insert the

data into the table if emp_dept field doesn’t allow nulls.

• Delete anomaly: Suppose, if at a point of time the

company closes the department D890 then deleting the

rows that are having emp_dept as D890 would also delete

the information of employee Maggie since she is assigned

only to this department.

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• To overcome these anomalies we need to normalize the

data. In the next section we will discuss about

normalization.

• Normalization

• Here are the most commonly used normal forms:

• First normal form(1NF)

• Second normal form(2NF)

• Third normal form(3NF)

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First normal form (1NF)

• As per the rule of first normal form,

an attribute (column) of a table cannot hold multiple values.

It should hold only atomic values.

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• Example: Suppose a company wants to store the names

and contact details of its employees. It creates a table that

looks like this:

emp_id emp_name emp_address emp_mobile

101 Herschel New Delhi 8912312390

102 Jon Kanpur 8812121212 9900012222

103 Ron Chennai 7778881212

104 Lester Bangalore 9990000123 8123450987

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• Two employees (Jon & Lester) are having two mobile

numbers so the company stored them in the same field as

you can see in the table below.

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• This table is not in 1NF as the rule says

“each attribute of a table must have atomic (single) values”,

• the (emp_mobile) values for employees Jon & Lester

violates that rule

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• To make the table complies with 1NF we should have the

data like this:

emp_id emp_name emp_address emp_mobile

101 Herschel New Delhi 8912312390

102 Jon Kanpur 8812121212

102 Jon Kanpur 9900012222

103 Ron Chennai 7778881212

104 Lester Bangalore 9990000123

104 Lester Bangalore 8123450987

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Second normal form (2NF)

A table is said to be in 2NF if both the following

conditions hold:

• Table is in 1NF (First normal form)

• No non-prime attribute is dependent on the proper subset

of any candidate key of table.

• An attribute that is not part of any candidate key is known

as non-prime attribute.

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• Example: Suppose a school wants to store the data of

teachers and the subjects they teach. They create a table

that looks like this: Since a teacher can teach more than

one subjects, the table can have multiple rows for a same

teacher.

teacher_id subject teacher_age

111 Math 38

111 Physics 38

222 Biology 38

333 Physics 40

333 Chemistry 40

Candidate Keys:

{teacher_id, subject}

Non prime attribute:

teacher_age

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• The table is in 1 NF because each attribute has atomic

values

• However, it is not in 2NF because non-prime attribute

(teacher_age) is dependent on teacher_id alone.

• This violates the rule for 2NF as the rule says

• “NO non-prime attribute is dependent on the proper

subset of any candidate key of the table”.

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• To make the table complies with 2NF we can break it in two

tables like this:

• teacher_subject table: teacher_details table:

teacher_id teacher_age

111 38

222 38

333 40

teacher_id subject

111 Maths

111 Physics

222 Biology

333 Physics

333 Chemistry

Now the tables comply with Second normal form (2NF).

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Third Normal form (3NF)

A table design is said to be in 3NF if both the following conditions hold:

• Table must be in 2NF

- An attribute that is not part of any candidate key is known as non-prime attribute.

• A table is in 3NF if

• it is in 2NF and

• at least one of the following conditions hold:

• X→ Y

• X is a super key of table

• Y is a prime attribute of table

An attribute that is a part of one of the candidate keys is known as prime attribute.

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https://beginnersbook.com/2015/04/candidate-key-in-dbms/

https://beginnersbook.com/2015/04/super-key-in-dbms/

• Example: Suppose a company wants to store the complete address of each employee, they create a table named employee_details that looks like this:

emp_id emp_name emp_zip emp_state emp_city emp_district

1001 John 282005 UP Agra Dayal Bagh

1002 Ajeet 222008 TN Chennai M-City

1006 Lora 282007 TN Chennai Urrapakkam

1101 Lilly 292008 UK Pauri Bhagwan

1201 Steve 222999 MP Gwalior Ratan

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Super keys: {emp_id}, {emp_id, emp_name},

{emp_id, emp_name, emp_zip}…so on

Candidate Keys: {emp_id}

• Non-prime attributes: all attributes except emp_id are non-prime as they are not part of candidate key.

• Here, (emp_state, emp_city & emp_district )dependent on (emp_zip).

• And, (emp_zi)p is dependent on (emp_id) that makes

non-prime attributes (emp_state, emp_city & emp_district)

transitively dependent on super key (emp_id). This violates

the rule of 3NF.

• To make this table complies with 3NF we have to break the table into two tables to remove the transitive dependency:

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• Employee table:

emp_id emp_name emp_zip

1001 John 282005

1002 Ajeet 222008

1006 Lora 282007

1101 Lilly 292008

1201 Steve 222999

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• employee_zip table:

emp_zip emp_state emp_city emp_district

282005 UP Agra Dayal Bagh

222008 TN Chennai M-City

282007 TN Chennai Urrapakkam

292008 UK Pauri Bhagwan

222999 MP Gwalior Ratan

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RELATIONAL ALGEBRA

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Documents

Normalization in DBMS: 1NF, 2NF, 3NF and BCNF in Database 07... · 2021. 2. 15. · instances of an entity. It acts as a cross-reference between two tables as it references the primary