Question $4.$ Let Books be a ternary relation, where the first position is a book's

title, the second is the name of its author, and the third is its length $($number of

pages$).$

$($a$)$ Express the constraint "a book's length is a positive quantity" as a Boolean

query in safe Relational Calculus, without using existential quantification.

$($b$)$ Write a Boolean query in safe Relational Calculus that returns true if and

only if $"$no two books of the same title and by the same author have different

lengths".

$($c$)$ Write a query in safe Relational Calculus that returns the name of each au$-$

thor whose books are all under $200$ pages long $($each$).$