SQL statement to prove that A->B in a R(ABCD)

Question

How do I write a SQL statement that proves the functional dependency A → B holds given a relation with attributes ABCD knowing that no record has NULL values?

Answer 1

You could also group by the functional dependency you want to check (in this case A->B), and count if there are distinct B´s for each A.

SELECT *  
FROM R 
GROUP BY A 
HAVING COUNT (DISTINCT B) > 1;

If you would like to check if A -> B,C you could expand the query:

SELECT *  
FROM R 
GROUP BY A 
HAVING COUNT (DISTINCT B) > 1 OR COUNT (DISTINCT C)>1;