Database miscellaneous Easy Questions and Answers

Database miscellaneous

Consider a join (relation algebra) between relations r (R)and s (S) using the nested loop method. There are 3 buffers each of size equal to disk block size, out of which one buffer is reserved for intermediate results. Assuming size (r (R))< size(s (S)), the join will have fewer number of disk block accesses if

1. relation r (R) is in the outer loop.
1. relation s (S) is in the outer loop.
1. join selection factor between r (R) and s (S) is more than 0.5.
1. join selection factor between r (R) and s (S) is less than 0.5.

Consider two relations R₁ (A,B) with the tuples(1, 5), (3, 7) and R₂ (A, C) = (1, 7), (4, 9). Assume that R(A,B,C) is the full natural outer join of R₁ and R₂ . Consider the following tuples of the form (A, B, C): a = (1, 5, null), b = (1, null, 7), c = (3, null, 9), d = (4, 7, null), e = (1, 5, 7), f = (3, 7, null), g = (4, null, 9). Which one of the following statements is correct?

1. R contains a, b, e, f, g but not c, d.
1. R contains all of a, b, c, d, e, f, g.
1. R contains e, f, g but not a, b.
1. R contains e but not f, g.

Which of the following is NOT a superkey in a relational schema with attributes V,W, X, Y, Z and primary key V Y?

Consider a database that has the relation schemas EMP(EmpId, EmpName, DeptId), and DEPT(DeptName, DeptId). Note that the DeptId can be permitted to be NULL in the relation EMP. Consider the following queries on the database expressed in tuple relational calculus.
(I) {t | ∀ u ∈ EMP(t[EmpName] = u[EmpName] ∧ v ∈ DEPT(t[DeptId] ≠ v[DeptId]))}
(II) {t | ∀ u ∈ EMP(t[EmpName] = u[EmpName] ∧ ∃ v ∈ DEPT(t[DeptId] ≠ v[DeptId]))}
(III) {t | ∀ u ∈ EMP(t[EmpName] = u[EmpName] ∧ ∃ v ∈ DEPT(t[DeptId] = v[DeptId]))}
Which of the above queries are safe?

1. (I) and (II) only
1. (I) and (III) only
1. (II) and (III) only
1. (I), (II) and (III)