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Functional dependency

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Topic

blanc-de-poulet
Starting Member

4 Posts

Posted - 2005-04-06 : 19:50:48

ok this is prolly the biggest noob question around but hey...
I m asked to write a statement to show that the data in R(A,B,C) supports the FD A -> B

and I wrote
SELECT count(*)
FROM R A1, R A2
WHERE a1.A=A2.A AND A1.B != A2.B;

which is wrong
Someone?

thanks,
JD

tkizer
Almighty SQL Goddess

38200 Posts

Posted - 2005-04-06 : 19:51:44

What does this mean: R(A,B,C) supports the FD A -> B?

Tara

blanc-de-poulet
Starting Member

4 Posts

Posted - 2005-04-06 : 21:10:39

show that the data in the relation R(A,B,C) supports the supposed Functional Dependency A -> B

blanc-de-poulet
Starting Member

4 Posts

Posted - 2005-04-06 : 21:15:30

Oh did i misinterpreted your answer?
hmm
I thought could answer the original question by
"If 2 tuples in R agree in their A attribute, then their B attribute should also be the same"

byrmol
Shed Building SQL Farmer

1591 Posts

Posted - 2005-04-06 : 21:59:18

Let's use a "real life" example...

Given R = Employee (EmployeeID PK, LastName, FirstName)

The FD: (EmployeeID -> LastName) is saying that for any value of EmployeeID there is just one LastName value. Or "If I know the EmployeeID, I must know the LastName". It also says that another EmployeeID may also have the same LastName.

So.... We need to write a query that verifies the saying "An EmployeeID value has only one LastName value"

If any rows are returned then the FD does not hold...
SELECT EmployeeID, COUNT(LastName) AS LastNameCNT
FROM Employees
GROUP BY EmployeeID
HAVING COUNT(LastName) != 1

DavidM

A front-end is something that tries to violate a back-end.

blanc-de-poulet
Starting Member

4 Posts

Posted - 2005-04-06 : 23:15:17

that is exactly what I was looking for. The xplanation is also perfectly clear.
Thank you

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