How to Represent Functional Dependencies in Relational Theory

relational-theory

I'm trying to understand how to represent what functional dependencies are:

A1, A2,…, An → B

If two tuples of R have the same attributes for A1, A2,…, An then they have the same value for the attributes of B.

I am not able to represent it abstractly speaking… Can you give me an example?

Best Answer

Not sure if this is such a brilliant example, but assume a relation like:

CREATE TABLE OWNERSHIP
( item_id
, ssn
, firstname
,    unique(item_id, ssn) );

Assume the f.d.: 

ssn -> firstname

For the tuples:

(1, 'X', 'a name')  
(2, 'X', 'a name')

We can see that X in both cases has the name 'a name'.

Another way of thinking about it is: Assume two tuples of R have the same value of A but different values for B. Then by definition B can't be functionally dependent of A.

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