Database Normalization

Normalization forms
Normalization example
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Normalization forms
1st Normal Form
2nd Normal Form
3rd Normal Form
Boyce-Codd Normal Form
4th Normal Form
5th Normal Form
6th Normal Form

Key terms
Column – Attribute
Row – Tuple
Table – Relation
There are further enhancements to theory of normalization
and it is still being developed. There is even 6th normal
form but in any practical scenario, normalization achieves
its best shape in 3rd Normal form.
Entity – Any real world object that makes sense

Normalization example
Let us look at a library table that maintains all the books they rent
out in one single table:
Now let us push this data from various normal forms and see how we
can refine the data.

The rules of the first normal form are:
Each table cell should contain a single/atomic value
Every record in the table must be unique
1NF (First Normal Form)
Let us first convert the Books_Main_Table into 1NF

As per the 1NF rules, our Books Main Table looks good. Before we
proceed with 2NF and 3NF, we need to understand key columns.

Key / non-key columns
Any column (or group of columns) in a table which can uniquely identify each
row is known as key column. For example:
Phone number
Email ID
Student Roll Number
Employee ID
These are some columns which will always remain unique to every record inside
the table.
Such columns are known as key columns inside the table. Any column apart
from key columns is known as non-key column.

Primary key
A primary key is a single column value which uniquely identifies each
record in a table. In RDBMS, primary key must satisfy below:
Primary key must be unique
Primary key cannot be null
Every record will have primary key value

Composite Key
Sometimes its hard to define unique records with one single column. In
such cases, we can have two or more columns that uniquely identify each
record in a table. Such columns are known as composite key. For
example:
Name + Address
First Name + DOB + Father Name
Now that we know about key / non-key columns, let us move to 2NF.

2NF (Second Normal Form)
The rules of the second normal form are:
Table must be in 1NF
Every non-key attribute must be fully dependent on key attributes
We see that our Books_Main_Table does not have any primary key, in such
cases, we will have to introduce a new key column like Membership ID.
To make Books_Main_Table into 2NF, we need to see how columns are
closely related:
Membership ID has a salutation, name and address
Membership ID has books issued on their name

With this logic in mind, we will have to divide our Books_Main_Table
into two table:

Foreign Key
If you see the above tables, we have Membership ID in both tables
but in Membership_Details_table, it is a primary key column and
in Books_Issued_table, it is a non-key column.
Till now we have seen Primary key and composite key. A foreign
key refers to a primary key column of another table. This helps in
connecting two tables (and defines a relation between two tables).
A foreign key must satisfy below:
Foreign key column name can be different than primary key
column name
Unlike primary key, then need not be unique (see
Books_Issued_Table above)

Foreign key column can be null even though primary key
column cannot

Reason for Foreign key
When a user tries to insert a record into Books_Issued_Table
and if there is no membership ID exists in
Membership_Details_Table, it will be rejected. This way, we
maintain data integrity in RDBMS.
If there is no record with Membership ID in the parent table,
it will be rejected and database will throw an error.

3NF (Third Normal Form)
The rules of the third normal form are:
Data must be in 2NF
No transitive functional dependencies
What is transitive dependency?
In simple terms, if changing a non-key column causes any other non-key
column to change, then its called as transitive dependency.

In our example, if we change Full Name of the customer, it
might change Salutation

Final 3NF Tables
To move the Membership_Details_Table into 3NF, we need to further
divide the table into below:

We have divided the Membership_Details_Table
into a new Salutation_table.

Assignment
If you see the Books_Issued_Table, it still does not have
a key column. What do you think should be the key
column for the Books_Issued_Table? Or do we need to
introduce a new column?

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