Full Functional Dependency in Database Normalization

Hands of a web designer using a digital tablet, he is working with a desktop computer.
  Kohei Hara/Getty Images

A full functional dependency is a state of database normalization that equates to the normalization standard of Second Normal Form (2NF). In brief, this means that it meets the requirements of First Normal Form (1NF), and all non-key attributes are fully functionally dependent on the primary key.

This is not as complicated as it may sound. Let's look at this in more detail.

Summary of First Normal Form

Before a database can be fully functionally dependent, it must first comply with First Normal Form.

All this means that each attribute must hold a single, atomic value. 

For example, the following table does not comply with 1NF, because the employee Tina is linked to two locations, both of them in a single cell:

First Normal Form Non-Compliance
 Employee   Location
 John Los Angeles
 Tina Los Angeles, Chicago 






Allowing this design could negatively impact data updates or entries. To ensure 1NF compliance, rearrange the table so that all attributes (or column cells) hold a single value: 


First Normal Form Compliance

 Employee  Location John Los Angeles  Tina Los Angeles Tina Chicago








But 1NF is still not enough to avoid problems with the data.

How 2NF Works to Ensure Full Dependency

To be fully dependent, all non-candidate key attributes must depend on the primary key. (Remember, a candidate key attribute is any key (for example, a primary or foreign key) used to uniquely identify a database record.

Database designers use a notation to describe the dependent relationships between attributes:

If attribute A determines the value of B, we write this A -> B — meaning that B is functionally dependent on A. In this relationship, A determines the value of B, while B depends on A.

For example, in the following Employee Departments table, EmployeeID and DeptID are both candidate keys: EmployeeID is the table's primary key while DeptID is a foreign key.

Any other attribute — in this case, EmployeeName and DeptName — must depend on the primary key to obtain its value.

Employee Departments
 EmployeeID  EmployeeName  DeptID DeptName 
 Emp1 John Dept001 Finance
 Emp2 Tina Dept003 Sales
 Emp3 Carlos Dept001 Finance







In this case, the table is not fully dependent because, while the EmployeeName depends on the primary key EmployeeID, the DeptName depends instead on the DeptID. This is called partial dependency.

To make this table conform to 2NF, we need to separate the data into two tables:

 EmployeeID  EmployeeName  DeptID
 Emp1 John Dept001
 Emp2 Tina Dept003
 Emp3 Carlos Dept001







We remove the DeptName attribute from the Employees table and create a new table Departments:

 DeptID DeptName
 Dept001 Finance
 Dept002 Human Resources
 Dept003 Sales







Now the relations between the tables are fully dependent, or in 2NF.

Why Full Dependency Is Important

Full dependency between database attributes helps ensure data integrity and avoid data anomalies.

For example, consider the table in the section above that adheres only to 1NF. Here it is, again:

First Normal Form Compliance
 Employee Location
 John Los Angeles
 Tina Los Angeles
 Tina Chicago








Tina has two records. If we update one without realizing that there are two, the result would be inconsistent data. 

Or, what if we want to add an employee to this table, but we don't yet know the Location? We might be disallowed to even add a new employee if the Location attribute does not allow NULL values.

Full dependency is not the whole picture, though, when it comes to normalization. You must make sure that your database is in Third Normal Form (3NF).