What is Second Normal Form?
For a table to be in the Second Normal Form, it must satisfy two conditions:
- The table should be in the First Normal Form.
- There should be no Partial Dependency.
If you want you can skip the video, as the concept is covered in detail below the video.
What is Partial Dependency? Do not worry about it. First let's understand what is Dependency in a table?
What is Dependency?
Let's take an example of a Student table with columns
address(student's home address).
In this table,
student_id is the primary key and will be unique for every row, hence we can use
student_id to fetch any row of data from this table
Even for a case, where student names are same, if we know the
student_id we can easily fetch the correct record.
Hence we can say a Primary Key for a table is the column or a group of columns(composite key) which can uniquely identify each record in the table.
I can ask from branch name of student with
student_id 10, and I can get it. Similarly, if I ask for name of student with
student_id 10 or 11, I will get it. So all I need is
student_id and every other column depends on it, or can be fetched using it.
This is Dependency and we also call it Functional Dependency.
What is Partial Dependency?
Now that we know what dependency is, we are in a better state to understand what partial dependency is.
For a simple table like Student, a single column like
student_id can uniquely identfy all the records in a table.
But this is not true all the time. So now let's extend our example to see if more than 1 column together can act as a primary key.
Let's create another table for Subject, which will have
subject_name fields and
subject_id will be the primary key.
Now we have a Student table with student information and another table Subject for storing subject information.
Let's create another table Score, to store the marks obtained by students in the respective subjects. We will also be saving name of the teacher who teaches that subject along with marks.
In the score table we are saving the student_id to know which student's marks are these and subject_id to know for which subject the marks are for.
student_id + subject_id forms a Candidate Key(learn about Database Keys) for this table, which can be the Primary key.
Confused, How this combination can be a primary key?
See, if I ask you to get me marks of student with
student_id 10, can you get it from this table? No, because you don't know for which subject. And if I give you
subject_id, you would not know for which student. Hence we need
student_id + subject_id to uniquely identify any row.
But where is Partial Dependency?
Now if you look at the Score table, we have a column names
teacher which is only dependent on the subject, for Java it's Java Teacher and for C++ it's C++ Teacher & so on.
Now as we just discussed that the primary key for this table is a composition of two columns which is
subject_id but the teacher's name only depends on subject, hence the
subject_id, and has nothing to do with
This is Partial Dependency, where an attribute in a table depends on only a part of the primary key and not on the whole key.
How to remove Partial Dependency?
There can be many different solutions for this, but out objective is to remove teacher's name from Score table.
The simplest solution is to remove columns
teacher from Score table and add it to the Subject table. Hence, the Subject table will become:
And our Score table is now in the second normal form, with no partial dependency.
- For a table to be in the Second Normal form, it should be in the First Normal form and it should not have Partial Dependency.
- Partial Dependency exists, when for a composite primary key, any attribute in the table depends only on a part of the primary key and not on the complete primary key.
- To remove Partial dependency, we can divide the table, remove the attribute which is causing partial dependency, and move it to some other table where it fits in well.