Suppose relation R (A,B) has the tuples : Choose the dangling tuple(s) for the…
Suppose relation R (A,B) has the tuples :
Choose the dangling tuple(s) for the relational algebra given below.
Answer: A. (4,7,9) of S; B. (1,2) of R — Dangling tuples: (1,2) from R and (4,7,9) from S. Why (1,2) from R is dangling: R.B = 2, and there is no S tuple with D = 2, so no S tuple satisfies R.B = S.D…
- A.
(4,7,9) of S
- B.
(1,2) of R
- C.
(4,6,8) of S
- D.
There are no dangling tuples
Attempted by 7 students.
Show answer & explanation
Correct answer: A, B
Dangling tuples: (1,2) from R and (4,7,9) from S.
Why (1,2) from R is dangling: R.B = 2, and there is no S tuple with D = 2, so no S tuple satisfies R.B = S.D (and thus no tuple satisfies both required conditions).
Why (4,7,9) from S is dangling: S.D = 7, and no R tuple has B = 7, so no R tuple can match this S tuple under R.B = S.D (and the R.A > S.C check is irrelevant once the equality fails).
Examples of tuples that do match:
R tuple (3,4) matches S tuple (2,4,6) because R.B = 4 = S.D and R.A = 3 > S.C = 2.
R tuple (5,6) matches S tuple (4,6,8) because R.B = 6 = S.D and R.A = 5 > S.C = 4.