DMDB 1

Keys enforce uniqueness

• True

• False

Answer explanation

This is true. A (Candidate) Key is a minimal set of fields that identify each tuple uniquely.

R₁ ⋃ R₂ defines the ... of relations R₁ and R₂

Answer explanation

This describes the union of R₁ and R₂

Choose the correct relational algebra operator which corresponds to selection

• σ

• s

• Π

• ρ

• ᵩ

Answer explanation

σ (sigma) stands for selection. In Relational Algebra, selection returns tuples which satisfy a given condition

• Difference

• Union

Answer explanation

This is the definition of the difference

Π stands for...

Answer explanation

Projection only keeps a subset of columns of some relation.

Hint to remember: Π represents "Pi" and Projection starts with P

The intersection in relational algebra can be defined only by the difference:

• No

Answer explanation

If A contains 2 tuples and B contains 4 tuples, then A X B contains ... tuples

• 2

• 4

• 8

• 6

• Cannot be said, depends on A and B

Answer explanation

Note that A and B are sets. The cartesian/cross product A X B contains |A| * |B| tuples (recall from discrete maths)

To rename attribute a to f in relation R we write...

• ρ_a->f(R)

• δ_a->f(R)

• σ_a->f(R)

• R(φ(a,f))

• R_a->f

Answer explanation

Don't be fooled. Renaming has the "rho" (ρ) symbol.

𝑅₁(𝐴, 𝐵) ⋈ 𝑅₂ (𝐵, 𝐶) =

• Π_{𝐴,𝐵,𝐶}(𝜎_{𝑅1.𝐵=𝑅2.𝐵} (𝑅₁ × 𝑅₂) )

• Π_{𝐴,𝐵,𝐶}(𝜎_{𝑅1.A=𝑅2.𝐵} (𝑅₁ × 𝑅₂) )

• 𝜎_{𝐴,𝐵,𝐶}(Ρ_{𝑅1.A=𝑅2.𝐵} (𝑅₁ × 𝑅₂) )

• Π_𝑅1−R2𝑅1 − Π_𝑅1−R2 ((Π_𝑅1−R2𝑅1) × R1 − 𝑅2)

• Π_𝑅2−R1𝑅2 − Π_𝑅1−R2 ((Π_𝑅1−R2𝑅1) × R2 − 𝑅1)

Answer explanation

This is the represention of a natural join.

Select what holds true for relational division

• 𝑅 ÷ 𝑆 = 𝑇 : 𝑇 is the largest relation such that 𝑆 × 𝑇 ⊆ R

• 𝑅 ÷ 𝑆 = 𝑇 : 𝑇 is the smallest relation such that 𝑆 × 𝑇 ⊆ R

• 𝑅 ÷ 𝑆 = Π_𝑅−𝑆𝑅 − Π_𝑅−𝑆 ((Π_𝑅−𝑆𝑅) × 𝑆 − 𝑅)

• Relational division 𝑅 ÷ 𝑆 is equivalent to the natural join 𝑅 ⋈ 𝑆

Answer explanation

This is tricky!