CODEWITHSUNDEEP
joinrelational-databaserelational-algebra
Maggie Liu
Maggie Liu

Reputation: 366

Relational Algebra Stanford Lagunitas Online Course Quiz

Problem: Compute the natural join of R and S. Which of the following tuples is in the result? Assume each tuple has schema (A,B,C,D).

Relation R 

| A | C |
|---|---|

| 3 | 3 |
| 6 | 4 |
| 2 | 3 |
| 3 | 5 |
| 7 | 1 |

Relation S

| B | C | D |
|---|---|---|
| 5 | 1 | 6 |
| 1 | 5 | 8 |
| 4 | 3 | 9 |

I'm not quite sure what it means by "assume each tuple has a schema of A,B,C,D". Does this mean the R relation has a scheme of ABCD although it only lists A and C? I should assume there's also B and D but columns B and D are blank?

Operating under that assumption, I got the answer wrong. The explanation says there's no (7,5) in R which there clearly is under column A. Could someone explain to me what I'm doing wrong or if I'm missing something? Thank you!

Upvotes: 1

Views: 389

Answers (2)

Aaron Dietz
Aaron Dietz

Reputation: 10277

The answer feedback is misleading and wrong, that would be the feedback if you choose (7,1,5,8)

Your answer is right.

For thoroughness: in a natural join you connect tuples on common attributes, in this case C is the attribute in common.

Your return tuples are:

  R        S
 A,C     B,C,D     A,B,C,D
(7,1) & (5,1,6) = (7,5,1,6)
(3,5) & (1,5,8) = (3,1,5,8)
(2,3) & (4,3,9) = (2,4,3,9)
(3,3) & (4,3,9) = (3,4,3,9) --Your answer, correct

I even found a Stanford doc defining a natural join, just in case they lived in a different universe than the rest of us, but they don't. It's just a bug in the quiz.

Upvotes: 1

philipxy
philipxy

Reputation: 15118

The question doesn't say R has that scheme. It says the natural join of R & S has that scheme.

(There are many variations on what a relation is, what relational operators are available, how they work & what their symbols are. They are telling you to expect that the schema for the join of those two relations has columns A, B, C & D. You should already know that from the definitions in the course, but since they give it nobody should get that part wrong.)

You seem to be saying that your choice of a row in the natural join was 2. That's correct. The explanation says that a wrong choice can't be right because tuple (7,5) is not in R. They do not mean that (7,5) is a list of values "under column A". But that feedback is for choice 3, not choice 2. So the answer checking seems to have a bug. Let them know.

Upvotes: 1

Related Questions