Let R₁ (A, B, C) and R₂ (D, E) be two relation schema, where the primary keys are shown underlined, and let C be a foreign key in R₁ referring to R₂. Suppose there is no violation of the above referential integrity constraint in the corresponding relation instances r₁ and r₂. Which one of the following relational algebra expressions would necessarily produce an empty relation?

Consider the following SQL query:

Select distinct a1, a2, ..., an 

From r1, r2, ..., rm 

Where P; 

For an arbitrary predicate P, this query is equivalent to which of the following relational algebra expressions?

With regard to the expressive power of the formal relational query languages, which of the following statements is true?

Given the relations

employee (name, salary, deptno), and
department (deptno, deptname, address)

Which of the following queries cannot be expressed using the basic relational algebra operations $$\left(\sigma, \pi,\times ,\Join, \cup, \cap,-\right)$$?