Question

Suppose a relation R(A,B) and S(B,C) are bags. R has 2 tuples (1,3), (2,4) and S has 3 tuples (1,2), (1,2) and (4,5). The theta join R ⋈ R.B>S.B S has __________ tuples

Select one:
a. 2
b. 3
c. 6
d. 4

Answer 1

The theta join of relations R and S where R.B > S.B results in 2 tuples, (2,4) joined with (1,2) twice, considering bag semantics.

Step-by-step explanation:

The question is asking for the number of tuples resulting from a theta join on relation R and S where R.B is greater than S.B. Given the tuples in R (1,3) and (2,4), and the tuples in S (1,2), (1,2), and (4,5), we observe that the condition for the theta join (R.B > S.B) is satisfied in the following cases:

• (2,4) in R and (1,2) in S satisfy the condition 4 > 2.
• (2,4) in R and the second (1,2) in S again satisfy the condition 4 > 2.

Since both instances of the tuple (1,2) in S are considered separate due to the bag (or multiset) semantics, we count each occurrence separately when joining with R. Therefore, we have two tuples that satisfy the condition.

In a theta join, the tuples from the two relations are combined based on a specified condition. In this case, we need to find the tuples in relation R where the value of 'B' is greater than the value of 'B' in relation S.

From the given data, we can see that the tuples with 'B' values greater than 2 in relation R are (2,4). Therefore, the theta join R ⋈ R.B>S.B S will have 1 tuple (2,4).

The correct answer to the question is: 2 tuples.