Given relations r( w, x ) and s( y, z ), the result of

select distinct w,x
from r, s;

is guaranteed to be same as r, provided

In SQL, relations can contain null values, and comparisons with null values are treated as unknown. Suppose all comparisons with a null value are treated as false. Which of the following pairs is not equivalent?

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 $$\left(\sigma, \pi,\times ,\Join, \cup, \cap,-\right)$$?

B⁺-trees are preferred to binary trees in databases because