Semantically Obvious FDs

Main Entry: semantic 
Function: adjective
1 : of or relating to meaning in language

Merriam-Webster Dictionary

How to Decide If an FD is Semantically Obvious

1. Write what (you think) the FD means as an English statement.

2. Using the information you have available to you, decide whether it seems reasonable for the statement to be true.

Worked Example

Below are two functional dependencies (FDs) for attributes in a model of the University of Woolloomooloo.

For each FD:

State whether or not the FD is Semantically Obvious.

First FD

StudentId  →  StudentName

1. Only one student name can be associated with each Student Id at any one time.

2. Based on our real world knowledge of how universities operate, it seems reasonable for this to be true. Where students have more than one name over time, one student name is the StudentName and the other(s) may be PreviousName(s).

Semantically Obvious

Second FD

StudentId, CourseCode  →  Grade

1. A student can receive only one Grade for each course s/he takes at the University of Woolloomooloo

2. We know that universities allow students to repeat courses (to enrol in more than one offering of a course), and we know that universities record the results obtained by all students for all course offerings

Not Semantically Obvious

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