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
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




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