Computer Sciene of Udayana State University

April 26, 2009

Functional Dependency (FD)

Filed under: Computer Science — ignaga @ 2:27 AM

u Important concept for normalization.

u Functional Dependency (FD)

Are constraints that are derived from the meaning and interrelationships of the data attributes

If A and B are attributes of relation R, B is functionally dependent on A (denoted A –> B), or A functionally determines B, if each value of A in R is associated with exactly one value of B in R.

Each value of A determines a value of B

A and B can be sets of attributes.

u FDs are derived from the real-world constraints on the attributes

u Social security number determines employee name

SSN –> ENAME

u Project number determines project name and location

PNUMBER –> {PNAME, PLOCATION}

u Employee ssn and project number determines the hours per week that the employee works on the project

{SSN, PNUMBER} –> HOURS

u An FD is a property of the attributes in the schema R.

u The constraint must hold on every relation instance.

u If K is a key of R, then K functionally determines all attributes in R

(since we never have two distinct tuples with t1[K]=t2[K])

u FDs are nontrivial. (StaffNo–>StaffNo is trivial)

example

Given TEXT we know the COURSE.

TEXT –> COURSE

u TEXT maps to a single value of COURSE

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1 Comment »

  1. […]   Associated Article: Computer Sciene of Udayana State University […]

    Pingback by Practical Dependency (FD) | Ragnarok Connection — July 16, 2014 @ 5:05 AM | Reply


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