Slide 16.15: Functional dependencies (FDs)

Functional Dependencies (FDs)

Functional dependency is a relationship that exists when one attribute uniquely determines another attribute. FD helps to maintain the quality of data in the database, and is a foundation of database normalization.

Definition of Functional Dependencies
Give a table T containing two attributes A and B, A→B, or A functionally determines B if

The attribute(B) is functionally dependent on attribute (A) if each value in column (A) determines one and only one value in column (B); or

Given two rows r₁ and r₂ in a table T, if r₁(A)=r₂(A) then r₁(B)=r₂(B).

Graphical representation of functional dependencey is given on the right:

An Example
The previous single table designed is given on the right again:

emp_info
emp_id	emp_name	...	skill_id	skill_name	skill_date	skill_lvl
09112	Jones	...	44	librarian	03-15-20	12
09112	Jones	...	26	typing	06-30-18	10
09112	Jones	...	89	word-proc	01-15-16	12
12231	Smith	...	26	typing	04-15-19	5
12231	Smith	...	39	filing	07-30-21	7
...	...	...	...	...	...	...

FDs are based on the intent of the database designers. From the table content, the table’s FDs are given on the right (note that this is unusual because it is reverse thinking), where ↛ means “does not functionally determine.”

  emp_id → emp_name
  emp_id → emp_phone
  emp_id → dept_name
  emp_phone ↛ emp_id