Normalization is a systematic approach of decomposing tables to eliminate data redundancy and undesirable characteristics like Insertion, Update and Deletion Anamolies. It is a two step process that puts data into tabular form by removing duplicated data from the relation tables.

Normalization is used for mainly two purpose,

  • Eliminating reduntant(useless) data.
  • Ensuring data dependencies make sense i.e data is logically stored.

 

Without Normalization

Without Normalization, it becomes difficult to handle and update the database, without facing data loss. Insertion, Updation and Deletion Anamolies are very frequent if Database is not Normalized. To understand these anomalies let us take an example of Student table.
S_id  S_Name  S_Address  Subject_opted
401    Adam       Noida           Bio
402    Alex         Panipat        Maths
403    Stuart     Jammu         Maths
404    Adam     Noida            Physics
Updation Anamoly : To update address of a student who occurs twice or more than twice in a table, we will have to update S_Address column in all the rows, else data will become inconsistent.

Insertion Anamoly : Suppose for a new admission, we have a Student id(S_id), name and address of a student but if student has not opted for any subjects yet then we have to insert NULL there, leading to Insertion Anamoly.

Deletion Anamoly : If (S_id) 401 has only one subject and temporarily he drops it, when we delete that row, entire student record will be deleted along with it.
Normalization Rule

Normalization rule are divided into following normal form.

  • First Normal Form
  • Second Normal Form
  • Third Normal Form
  • BCNF

First Normal Form (1NF)

A row of data cannot contain repeating group of data i.e each column must have a unique value. Each row of data must have a unique identifier i.e Primary key. For example consider a table which is not in First normal form

Student Table :
S_id     S_Name      subject
401       Adam           Biology
401       Adam            Physics
402      Alex               Maths
403      Stuart            Maths

You can clearly see here that student name Adam is used twice in the table and subject math is also repeated. This violates the First Normal form. To reduce above table to First Normal form break the table into two different tables

New Student Table :
S_id     S_Name
401       Adam
402      Alex
403      Stuart

Subject Table :
subject_id     student_id     subject
10                    401                   Biology
11                     401                   Physics
12                    402                   Math
12                    403                   Math

In Student table concatenation of subject_id and student_id is the Primary key. Now both the Student table and Subject table are normalized to first normal form
Second Normal Form (2NF)

A table to be normalized to Second Normal Form should meet all the needs of First Normal Form and there must not be any partial dependency of any column on primary key. It means that for a table that has concatenated primary key, each column in the table that is not part of the primary key must depend upon the entire concatenated key for its existence. If any column depends oly on one part of the concatenated key, then the table fails Second normal form. For example, consider a table which is not in Second normal form.

Customer Table :
customer_id       Customer_Name       Order_id       Order_name       Sale_detail
101                         Adam                                  10                      order1               sale1
101                        Adam                                   11                       order2              sale2
102                       Alex                                      12                       order3              sale3
103                       Stuart                                   13                       order4              sale4

In Customer table concatenation of Customer_id and Order_id is the primary key. This table is in First Normal form but not in Second Normal form because there are partial dependencies of columns on primary key. Customer_Name is only dependent on customer_id, Order_name is dependent on Order_id and there is no link between sale_detail and Customer_name.

To reduce Customer table to Second Normal form break the table into following three different tables.

Customer_Detail Table :
customer_id    Customer_Name
101                     Adam
102                    Alex
103                  Stuart

Order_Detail Table :
Order_id      Order_Name
10                    Order1
11                     Order2
12                     Order3
13                     Order4

Sale_Detail Table :
customer_id                Order_id            Sale_detail
101                                      10                         sale1
101                                      11                          sale2
102                                     12                          sale3
103                                    13                            sale4

Now all these three table comply with Second Normal form.
Third Normal Form (3NF)

Third Normal form applies that every non-prime attribute of table must be dependent on primary key. The transitive functional dependency should be removed from the table. The table must be in Second Normal form. For example, consider a table with following fields.

Student_Detail Table :
Student_id        Student_name      DOB      Street      city      State            Zip

In this table Student_id is Primary key, but street, city and state depends upon Zip. The dependency between zip and other fields is called transitive dependency. Hence to apply 3NF, we need to move the street, city and state to new table, with Zip as primary key.

New Student_Detail Table :
Student_id        Student_name        DOB           Zip

Address Table :
Zip     Street      city       state

The advantage of removing transtive dependency is,

Amount of data duplication is reduced.
Data integrity achieved.

Boyce and Codd Normal Form (BCNF)

Boyce and Codd Normal Form is a higher version of the Third Normal form. This form deals with certain type of anamoly that is not handled by 3NF. A 3NF table which does not have multiple overlapping candidate keys is said to be in BCNF.