Consider the following flight database relations. Primary key attributes are underlined.

The following relations keep track of airline flight information:

Flights: $($flno: integer, origin: string, destination: string, distance, integer, departs:

timestamp, arrives: timestamp, price: integer$)$

Aircraft$($aid: integer, aname: string, cruisingrange: integer$)$

Certified$($eid: integer, aid: integer$)$

Employees$($eid: integer, ename: string, salary: integer$)$

Note that the employees relation describes pilots and other kinds of employees as well;

every pilot is certified for some aircraft, and only pilots are certified to fly. Write each

of the following queries in SQL:

$($a$)$ List in reverse alphabetical order all pilots who are certified to fly some Airbus

plane.

$(5$ marks$)$

$($b$)$ Find the name$($s$)$ and salary$($salaries$)$ of the pilot$($s$)$ who is $($are$)$ certified to fly

the largest number of planes.

$(5$ marks$)$

$($c$)$ Compute the difference between the average salary of a pilot and the average

salary of all employees $($including pilots$).$

$(5$ marks$)$

$($d$)$ For each plane that has at least six pilots, find the name of the plane and the

average salary of the pilots who are certified to fly it$.$

$(5$ marks$)$

$($e$)$ Find the set of origins and destinations that can be reached by two hops that

can't be reached by a direct flight $($e$.$g$.,$ list "Pittsburgh" "Honolulu" if there's

no direct flight between Pittsburgh and Honolulu and there's a flight from

"Pittsburgh" to some city $"$X$"$ and then a flight from $"$X$"$ to "Honolulu"$).$