<blockquote>
  <p><strong>Possible Duplicate:</strong><br>
  <a href="https://cstheory.stackexchange.com/questions/6517/producing-more-functional-dependencies-fds-from-a-list-of-functional-dependenci">Producing more functional dependencies (FDs) from a list of functional dependencies (FDs)</a>  </p>
</blockquote>



<p>I am currently trying to understand functional dependencies and their properties. I am looking at an example, but when I try to apply the rules I can't see to get the same answers. The question is:</p>

<blockquote>
  <p>Question: Given a relation T(A,B, C,D)
  and functional dependencies AB -> C,
  BC -> D,CD -> A AD -> B What FD’s
  follow from the given FD’s?</p>
  
  <p>Answer: AD -> CD,AD -> BC,CD -> AB.</p>
</blockquote>

<p>When applying the properties of functional dependencies I can't even see how they get the first answer (AD -> CD). If anyone could give me an idea of how we get to the first answer, maybe I can solve the rest myself.</p>

<p>PS: This is NOT homework (as I already have the answer!), but I am revising for an important exam and I need to understand these concepts.</p>

<p>Thanks!</p>

CC BY-SA 3.0
2011-05-10T16:15:41.113Z
The thread was closed because someone thought it was homework, when I've made it clear that it is NOT homework because I already have the answers. Any help is appreciated.
Alex
CC BY-SA 3.0
2011-05-10T16:40:38.763Z
Instead of posting a new duplicated question, here we usually discuss the scope of the original one in [meta](http://meta.cstheory.stackexchange.com/), and see if there are agreements on reopening it. And this site is for research-level questions in theoretical computer science, normally not included undergrad course materials, whether they are homework or not.
Hsien-Chih Chang 張顯之
CC BY-SA 3.0
2011-05-10T16:42:04.683Z
cstheory is for research level questions in theoretical computer science. As you mentioned, this is an undergraduate level question, and thus out of scope. I recommend sites like http://math.stackexchange.com/ for undergraduate level questions.
Artem Kaznatcheev
CC BY-SA 3.0
2011-05-10T16:45:06.767Z
Thanks for letting me know, I didn't realise cstheory was only for research-level questions. Please feel free to help me nonetheless.
Alex
<db.databases>
Producing more functional dependencies (FDs) from a list of functional dependencies (FDs)
Alex
# Producing more functional dependencies (FDs) from a list of functional dependencies (FDs)

(Asked by: username_0 on 2011-05-10)

> **Possible Duplicate:**  
> [Producing more functional dependencies (FDs) from a list of functional dependencies (FDs)](https://cstheory.stackexchange.com/questions/6517/producing-more-functional-dependencies-fds-from-a-list-of-functional-dependenci)

I am currently trying to understand functional dependencies and their properties. I am looking at an example, but when I try to apply the rules I can't see to get the same answers. The question is:

> Question: Given a relation T(A,B, C,D)
> and functional dependencies AB -> C,
> BC -> D,CD -> A AD -> B What FD’s
> follow from the given FD’s?
>
> Answer: AD -> CD,AD -> BC,CD -> AB.

When applying the properties of functional dependencies I can't even see how they get the first answer (AD -> CD). If anyone could give me an idea of how we get to the first answer, maybe I can solve the rest myself.

PS: This is NOT homework (as I already have the answer!), but I am revising for an important exam and I need to understand these concepts.

Thanks!

**Tags:** db.databases
> **Comments:**
> **username_0**: The thread was closed because someone thought it was homework, when I've made it clear that it is NOT homework because I already have the answers. Any help is appreciated.  
> **username_1**: Instead of posting a new duplicated question, here we usually discuss the scope of the original one in [meta](http://meta.cstheory.stackexchange.com/), and see if there are agreements on reopening it. And this site is for research-level questions in theoretical computer science, normally not included undergrad course materials, whether they are homework or not.  
> **username_2**: cstheory is for research level questions in theoretical computer science. As you mentioned, this is an undergraduate level question, and thus out of scope. I recommend sites like http://math.stackexchange.com/ for undergraduate level questions.  
> **username_0**: Thanks for letting me know, I didn't realise cstheory was only for research-level questions. Please feel free to help me nonetheless.  

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## 0 answers

