What is a "functional dependency"

Question

Hi..... Ques: I am not understand What is a 'functional dependency' please explain and give a example... What is a 'functional dependency'? How does it relate to database table design?

Prabhu Raja · Answer

Hi, Primary keys and UNIQUE constraints are two examples of how functional dependencies are implemented in SQL. Functional dependency: DEPTNO -> DNAME, LOC DNAME -> DEPTNO, LOC SQL implementation: create table dept (deptno integer PRIMARY KEY,dname varchar2(10) UNIQUE, loc varchar2(10));

Aarti · Answer

Hi Manish, A functional dependency occurs when one attribute in a relation uniquely determines another attribute. This can be written A -> B which would be the same as stating "B is functionally dependent upon A." it also known as Database Dependency. Example: In a table listing employee characteristics including Social Security Number (SSN) and name, it can be said that name is functionally dependent upon SSN (or SSN -> name) because an employee's name can be uniquely determined from their SSN. However, the reverse statement (name -> SSN) is not true because more than one employee can have the same name but different SSNs.

Satyapriya Nayak · Answer

Hi Manish, A dependency occurs in a database when information stored in the same database table uniquely determines other information stored in the same table. You can also describe this as a relationship where knowing the value of one attribute (or a set of attributes) is enough to tell you the value of another attribute (or set of attributes) in the same table. A functional dependency occurs when you already meet the requirements for a functional dependency and the set of attributes on the left side of the functional dependency statement cannot be reduced any farther. For example, "{SSN, age} -> name" is a functional dependency, but it is not a full functional dependency because you can remove age from the left side of the statement without impacting the dependency relationship. Please refer the below links http://www.emunix.emich.edu/~khailany/files/Normalization.htm http://databases.about.com/od/specificproducts/a/Database-Dependency.htm Thanks

What is a "functional dependency"

Answers (3)