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!
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.