Find minimal candidate keys

Question

I have 2 sets

R = {A B C D}
H = {AB-> C , AB-> D, D-> B}

Want to find all minimal keys in R set

My answer for minimal keys is : { A D }

this is because

AB -> C and AB -> D then AB -> CD
since D -> B then AD is the minimal keys

when i check my answer with this site. the site giving wrong answer. can explain?

The site is clearly wrong. Edit your question, and include step-by-step how you got your answer. Also include the title of the textbook you're following. — Mike Sherrill 'Cat Recall', May 30 '14 at 09:45

score 0 · Accepted Answer · answered May 30 '14 at 09:44

0

The site says, "Set of found candidate-keys: {{A, C, F}, {B, C, F}}." That's clearly wrong; F isn't even in R.

In any case, your answer is incomplete. AD is one of two candidate keys.

answered May 30 '14 at 09:44

so only have two candidate keys in my set? – user3664490 May 30 '14 at 14:37
Yes, only two candidate keys, based on the functional dependencies H={AB->C , AB->D, D->B}. – Mike Sherrill 'Cat Recall' May 30 '14 at 14:45
isn't another set of candidate key is { A C } ? i keep do and redo again – user3664490 May 30 '14 at 14:47
No, AC isn't a candidate key. Follow my advice in my comment to your question. It will save you time and frustration, *and* you'll get better answers. (And probably *more* answers, too.) – Mike Sherrill 'Cat Recall' May 30 '14 at 14:52
i just know how to find A D, another 1 not sure .. can teach? – user3664490 May 30 '14 at 15:01
To find all the candidate keys, you compute the "closure of a set of functional dependencies". (You can Google that. It's probably off-topic here, because it would take a chapter-length answer.) – Mike Sherrill 'Cat Recall' May 30 '14 at 15:06

1 Answers1