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please help me to understand the answer to this exercise about multivalued dependencies in Relational Database Design, ok,the problem is as follows:

Consider the relation StudentInfo(sID, dorm, courseNum). Students typically live in several dorms and take many courses during college. Suppose the data does not capture which dorm(s) a student lived in when taking a specific course, i.e., all dorm-course combinations are recorded for each student. What are all of the multivalued dependencies for relation StudentInfo?

These are the posible answers:

a) sID ->> dorm

b) sID ->> courseNum

c) sID ->> dorm, sID ->> courseNum ->> correct

d) sID ->> dorm, sID ->> courseNum, dorm ->> courseNum

Now, the correct answer is c.

Somethings that I can't grasp are:

Is dorm ->> courseNum a multivalued dependency?

Why dorm ->> courseNum implies that all students in each dorm take the same set of courses?

A lot of thanks, sorry for my english and lastly, the question was taken from Stanford Lagunita

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byron perez
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  • In general I think if you are taking a class you should talk to your professor. Both of you will get more out of it. – Hogan Nov 21 '18 at 19:10
  • Yeah, but believe me, that is not an option for me. – byron perez Nov 21 '18 at 19:28
  • @Hogan That's a free online course. But @ byronperez that doesn't mean you can't contact the professor re help or helpers. – philipxy Nov 21 '18 at 21:20
  • So give the basic *definitions* of & *facts* re MVDs that you have been given that might apply. Give in your own words what a row in the relation says & the relation's constraints. See [ask] & hits googling 'stackexchange homework'--show what you can do or think may be relevant. Eg: Can you represent the relation in terms of smaller ones? A basic MVD fact says, when there is an MVD there is a certain decomposition. So decompositions suggest possible MVDs. Find decomposition(s) corresponding to MVD(s). PS A MVD is not a relation--so rephrase "the dorm ->> courseNum relation" to make sense. – philipxy Nov 21 '18 at 21:42
  • Hi. *Definitions* & *facts*. Besides those being necessary for you or anyone to solve the problem, they are necessary for us to refer to in justifying/explaining a solution in an answer post. So please don't expect us to go get them when you won't. Also there is no use in our presenting them wholesale yet again, that is just us rewriting your textbook or the dozens free online in pdf. Where you don't understand a (published academic) presentation's definition or fact or how to apply it, quote it & ask/explain re being stuck there. – philipxy Nov 21 '18 at 22:09

1 Answers1

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There is no relationship between dorm and courseNum -- they are independent.

I don't know how to answer your 2nd question. If I were to translate the question to English, it seems you are asking if it is defined that each student in a dorm takes a class why does that define that each student in a dorm takes a class.

dorm ->> courseNum

Means for each dorm map it to a class.

Hogan
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  • Yeah, sorry for my bad explanation, regardless, In the Stanford website it is said: `The third multivalued dependency in Answer 4 says all students in each dorm take the same set of courses.` and I guess that is an important consideration in order to understand this problem. Isn't it? Thanks a lot for your help. – byron perez Nov 21 '18 at 19:41
  • @byronperez There is a "relationship between dorm and courseNum" here. It is "for some [sID], student [sID] lived in dorm [dorm] & took course [courseNum] during college". It is represented by `project dorm, courseNum (StudentInfo)`. This answer also is not consistent with what "->>" & "MVD" mean. "dorm ->> courseNum" here states that each {sID,dorm} subrow value from this relation appears with every {sID, course} subrow value from this relation with the same sID. (And vice versa.) That has something to do with some things being "independent" but nothing here is clear enough to be helpful. – philipxy Nov 22 '18 at 04:53
  • @byronperez -- that is exactly what I understood. It is an important consideration and I was making it. My point is if all students in a dorm take the same class then there is that translates into there being a relationship between dorm and class. – Hogan Nov 22 '18 at 19:08
  • @byronperez & Hogan The problem/assignment is very poorly conceived & phrased. Eg they don't mean "all classes", they mean, all classes in the table taken by the given student. Ditto for "all dorms". The problem is that if one understands MVDs & one tolerates ill-considered & poorly-phrased writing then one can guess a lot of what the poor problem statement should be saying, and if one's understanding of MVDs is weak then one is apt to wrongly attribute difficulties in understanding or answering the poor problem statement to oneself. (My answer's in draft.) – philipxy Nov 23 '18 at 00:23