GATE CS Tuple Calculus and SQL questions

Consider a relation geq which represents "greater than or equal to", that is, $(x,y) \in$ geq only if $y \geq x$. create table geq ( ib integer not null, ub integer not null, primary key ib, foreign key (ub) references geq on delete cascade ); Which of the following is possible if tuple (x,y) is deleted?

Which of the following relational calculus expression is not safe?

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 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.

Which of the following is/are correct?

The relational algebra expression equivalent to the following tuple calculus expression: $\left\{t \mid t \in r \land \left(t[A] = 10 \land t[B]=20\right)\right\}$ is

Which of the following query transformations (i.e., replacing the l.h.s. expression by the r.h.s expression) is incorrect? R1 and R2 are relations, C1 and C2 are selection conditions and A1 and A2 are attributes of R1.