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.
Anomalies in DBMS
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:
| 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 |
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.
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.
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)
- Boyce & Codd normal form (BCNF)
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.
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 |
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 above.
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.
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 |
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.
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 | Maths | 38 |
| 111 | Physics | 38 |
| 222 | Biology | 38 |
| 333 | Physics | 40 |
| 333 | Chemistry | 40 |
Candidate Keys: {teacher_id, subject}
Non prime attribute: teacher_age
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 which is a proper subset of candidate key. 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”.
To make the table complies with 2NF we can break it in two tables like this:
teacher_details table:
| teacher_id | teacher_age |
| 111 | 38 |
| 222 | 38 |
| 333 | 40 |
teacher_subject table:
| teacher_id | subject |
| 111 | Maths |
| 111 | Physics |
| 222 | Biology |
| 333 | Physics |
| 333 | Chemistry |
Now the tables comply with Second normal form (2NF).
Third Normal form (3NF)
A table design is said to be in 3NF if both the following conditions hold:
- Table must be in 2NF
- Transitive functional dependency of non-prime attribute on any super key should be removed.
An attribute that is not part of any candidate key is known as non-prime attribute.
In other words 3NF can be explained like this: A table is in 3NF if it is in 2NF and for each functional dependency X-> Y at least one of the following conditions hold:
- 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.
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 |
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 any candidate keys.
Here, emp_state, emp_city & emp_district dependent on emp_zip. And, emp_zip 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:
employee table:
| emp_id | emp_name | emp_zip |
| 1001 | John | 282005 |
| 1002 | Ajeet | 222008 |
| 1006 | Lora | 282007 |
| 1101 | Lilly | 292008 |
| 1201 | Steve | 222999 |
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 |
Boyce Codd normal form (BCNF)
It is an advance version of 3NF that’s why it is also referred as 3.5NF. BCNF is stricter than 3NF. A table complies with BCNF if it is in 3NF and for every functional dependency X->Y, X should be the super key of the table.
Example: Suppose there is a company wherein employees work in more than one department. They store the data like this:
| emp_id | emp_nationality | emp_dept | dept_type | dept_no_of_emp |
| 1001 | Austrian | Production and planning | D001 | 200 |
| 1001 | Austrian | stores | D001 | 250 |
| 1002 | American | design and technical support | D134 | 100 |
| 1002 | American | Purchasing department | D134 | 600 |
Functional dependencies in the table above:
emp_id -> emp_nationality
emp_dept -> {dept_type, dept_no_of_emp}
Candidate key: {emp_id, emp_dept}
The table is not in BCNF as neither emp_id nor emp_dept alone are keys.
To make the table comply with BCNF we can break the table in three tables like this:
emp_nationality table:
| emp_id | emp_nationality |
| 1001 | Austrian |
| 1002 | American |
emp_dept table:
| emp_dept | dept_type | dept_no_of_emp |
| Production and planning | D001 | 200 |
| stores | D001 | 250 |
| design and technical support | D134 | 100 |
| Purchasing department | D134 | 600 |
emp_dept_mapping table:
| emp_id | emp_dept |
| 1001 | Production and planning |
| 1001 | stores |
| 1002 | design and technical support |
| 1002 | Purchasing department |
Functional dependencies:
emp_id -> emp_nationality
emp_dept -> {dept_type, dept_no_of_emp}
Candidate keys:
For first table: emp_id
For second table: emp_dept
For third table: {emp_id, emp_dept}
This is now in BCNF as in both the functional dependencies left side part is a key.
Mahak says
Are you sure, that the example you given for Third Normal form (3NF) is correct. I have doubt, In the employee table and employee_zip table you relate ZIP in both tables but what If two employes having the same zip which record will be fetched from the employee_zip table ??
Robert Luse says
If two employees have the same zip, they will share the row in the zip table. There does not need to be two rows in the zip table and indeed, there should not be two rows in the zip table.
harshal davane says
WRONG IF WE CREATE NEW ZIP TABLE THEN WE CAN SEARCH THERE ZIP BYE NAME ALSO ..
amit says
name is not a prime attribute because multiple students can have same name and each student may have a different zip
sagar -441124
sagar -345632
MUDASSIR AHMED says
In employee table there will be 2 employees with same zip code but in employee_zip table there will be 1 record related to that zip code.The tables are related by zip code.So only 1 record will be fetched from employee_zip table. Hope you get the answer.
Gulfam says
Hey Mahak, there is only one record for every ZIP.
ZIP in itself the complete address.
Steve says
Mahak, That is the point they are trying to make is that many employees could be related to 1 Zip record. There would only be 1 entry in the Zip table per zip, since that’s the key. That is the point of 3NF, is to denormalize the duplicate data in the Employee table. Good luck!
DeepeshChaudhari says
I think there is no issue related to emp_zip……
becouse if any two employee have same emp_zip then it it means that both employee live’s in same area and so then in employee_zip table there is one row of that zip……..
and data will be fetched from single row………………….
Robert Luse says
As part of Normalization, there will be only one row for the the zip, not two. If two employees have the same zip, they will both use the information for that zip in the zip table.
Omenesa says
We should imagine a case scenario where two employees have the same zip code but different emp districts or emp city, which record will be fetched in such a scenario.
MUDASSIR AHMED says
In BCNF “dept_no_of_emp” is also candidate key.
Kalpesh says
Hi there,
I have read whole article of Normalization and I must say, it a best explanation with examples.
Examples are very useful for better understating the concept. I am really very thankful to you for the blog.
Thank you.
Pushpa says
Hi Chaitanya,
The concept of normalization with example explained is very helpful. It helped me to understand it clearly.
Thanks for sharing.
Best Wishes,
Pushpa
aman says
This topic was not understandable from book .after reading this I finally got it. Thank u.
Sid says
How is teacher_I’d, subject be the candidate key? Subject is redundant and only teacher I’d shld be sufficient.
Harsh Rohila says
Consider teacher_id 111, it is having two different subjects maths and physics. So only teacher_id cannot determine the complete row. Therefore subject is also required.
Jaswinder says
Teacher I’d alone cannot be the Candidate key because there will be many entries for a particular teacher as teacher can teach multiple subjects .And to fulfill criteria of becoming candidate key there should be unique values.
Richard Kidd says
A candidate key should be able to UNIQUELY IDENTIFY a row in a table. In the case of the teacher table, their are two rows in the table that can be identified with the teacher_id 111. If we are given teacher_id 111, we cannot discern if we need the record for subject ‘Maths’ or the record for subject ‘physics’. Therefore, teacher_id is not sufficient to uniquely identify a row. Likewise, as there are two rows with the teacher_id 111 and the teacher_age 38, these are also insufficient. The only minimal combination of attributes that uniquely identify a given row is {teacher_id, subject}.
Tharun Kumar Sunku says
Superb explanation, Thank you for this valuable information
sandeep says
hi chaitanya,
you explained in a single table to partition into different tables so it is easy to understand but my doubt is to how to partition those tables so please provide some information about how to partition a table
And also one thing before using those keys it is better to briefly explain about the keys so it is easy to understand
Seunfunmi says
Very useful information. Thank you for this article. I read the textbook but did not understand. Now I understand 1NF and 2NF. I’m still not fully clear with the 3NF and the BCNF though. Pls anyone with more detailed information?
deepesh chaudhari says
best notes of dbms forever……love it
Keynan says
Hi
Very good explanation.
I have one question: dosen’t the example you gave on the BCNF(before the BCNF solution) also break the second rule? because non prime attributes depends on only subset of the candidate key? for an example: the dept_type and dept_no_of_emp are only depended on a subset of the candidate key which is emp_dept
Thanks
amit says
In first table they are dependent, that is the violation of the 3NF. That’s why we decomposed the table and in second table Emp_dept is super key or candidate key not a subset of candidate key
just like foreign key concept
Anugya says
thnku for the making me understand the concept of normalization.
Rajiv Rai says
Isn’t the attribute emp_zip also a candidate key(3NF example)? If yes then wouldnt it violate the 3NF rule in the next table?
PuddiMan says
I don’t understand the example in BCNF. There are 2 primary keys, emp_id and emp_dept. This violates 2NF rules, emp_nationality can be determined by only emp_id. So in the first place, it is not in 2nf, why proceed to bcnf process?
Someone care to explain/correct me please(if i’m wrong)
Ninja says
Thanks a lot … 2morrow is my exam and this post really helped me.. Thanks a lot….
Yegon francis says
Is it allowed to use two primary keys in a relationship table?
rahul says
you should more explain ,candidate key ,and super key.
it is very difficult to find