Functional Dependency is mainly known as Normalization. Professor Codd in 1970 was the first person to define it. There are three steps to this dependency. Normalization is used to eliminate the three types of anomalies which are insertion, deletion and update anomalies. The three normalization are what a database can suffer from. They also define what the database is about because of all the work that goes into it. The Insertion Anomaly is when some of the data cannot be added into the database. There are just certain areas of the database where information just cannot be added or else it would mess up the system or get you a big (OOOPS). Update an0omaly is when there is redundancy going on in the database which will cause changes to everything that is in the database. Instead of change one information and the database changing all of them, a person has to go and do it one step at a time. Deletion anomaly is deleting of some data that causes other information to be lost or deleted.
Example -1. Assume the following relation
Student-courses (Sid:pk, Sname, Phone, Courses-taken)
Where attribute Sid is the primary key, Sname is student name, Phone is student's phone number and Courses-taken is a table contains course-id, course-description, credit hours and grade for each course taken by the student. More precise definition of table Course-taken is :
Course-taken (Course-id:pk, Course-description, Credit-hours, Grade)
According to the definition of first normal form relation Student-courses is not in first normal form because one of its attribute Courses-taken is itself a table and is not a simple attribute. To clarify it more assume the above tables contain the data as shown below:

Student-courses
Sid Sname Phone Courses-taken
100 John 487 2454 St-100-courses-taken
200 Smith 671 8120 St-200-courses-taken
300 Russell 871 2356 St-300-courses-taken