CS374-Database-Systems/Advanced-SQL/hw5.sql

--
-- Name: Nicholas Tamassia
--
-- Write your queries below each comment. Please use good style (one clause
-- per line, JOIN syntax, indentation) and make sure all queries end with a
-- semicolon. When necessary, limit the output to the first 200 results.
--
-- DO NOT MODIFY OR DELETE ANY OTHER LINES!
--

-- -----------------------------------------------------------------------------
-- Connect to air database
\c air
-- -----------------------------------------------------------------------------

--
\echo
\echo Query #1
--
-- List all last names in the database that start with the letter M. Use the
-- lower() function to convert all last names to lowercase. Show how many times
-- each last name is used. Note: last names are in more than one table.
--
-- Schema: count, last_name
--  Order: count (descending), last_name

SELECT COUNT(*) AS count, last_name
FROM (
    SELECT lower(last_name) AS last_name FROM passenger
    UNION ALL
    SELECT lower(last_name) FROM account
    UNION ALL
    SELECT lower(last_name) FROM frequent_flyer
)
WHERE last_name LIKE 'm%'
GROUP BY last_name
ORDER BY count DESC, last_name
LIMIT 200;

--
\echo
\echo Query #2
--
-- Show the passengers who have flown out of Dulles (IAD) but have never flown
-- out of Orlando (MCO).
--
-- Schema: passenger_id, first_name, last_name
--  Order: passenger_id

SELECT passenger_id, first_name, last_name
FROM (
    SELECT passenger_id, first_name, last_name
    FROM passenger
        JOIN booking ON passenger.booking_id = booking.booking_id
        JOIN booking_leg ON booking.booking_id = booking_leg.booking_id
        JOIN flight ON booking_leg.flight_id = flight.flight_id
    WHERE departure_airport = 'IAD'
    EXCEPT
    SELECT passenger_id, first_name, last_name
    FROM passenger
        JOIN booking ON passenger.booking_id = booking.booking_id
        JOIN booking_leg ON booking.booking_id = booking_leg.booking_id
        JOIN flight ON booking_leg.flight_id = flight.flight_id
    WHERE departure_airport = 'MCO'
)
ORDER BY passenger_id
LIMIT 200;

--
\echo
\echo Query #3
--
-- Find the passengers who have not been issued a boarding pass, but whose
-- booked itinerary includes a domestic flight from Dhaka (depart airport
-- DAC, arrive in Bangladesh BD).
--
-- Schema: first_name, last_name, iso_country
--  Order: passenger_id

SELECT p.first_name, p.last_name, a.iso_country
FROM passenger AS p
JOIN booking AS b ON p.booking_id = b.booking_id
JOIN booking_leg AS bl ON b.booking_id = bl.booking_id
JOIN flight AS f ON bl.flight_id = f.flight_id
JOIN airport AS a ON f.arrival_airport = a.airport_code
WHERE f.departure_airport = 'DAC'
    AND a.iso_country = 'BD'
    AND NOT EXISTS (
        SELECT 1
        FROM boarding_pass
        WHERE boarding_pass.passenger_id = p.passenger_id
    )
ORDER BY p.passenger_id
LIMIT 200;

--
\echo
\echo Query #4
--
-- For each airport, find the percentage of flights that have departed over 10
-- minutes late. Note that in order to avoid integer division, you must cast at
-- least one of the values to a float. For example: CAST(late_flights AS float)
-- Also, you will need to use the interval data type to represent 10 minutes.
--
-- Schema: departure_airport, percentage (calculated value)
--  Order: departure_airport

SELECT
    f.departure_airport,
    100.0 * CAST(SUM(
        CASE WHEN actual_departure > scheduled_departure + INTERVAL '10 minutes'
             THEN 1 ELSE 0 END
    ) AS float)
    /
    COUNT(*) AS percentage
FROM flight f
GROUP BY f.departure_airport
ORDER BY f.departure_airport
LIMIT 200;


-- -----------------------------------------------------------------------------
-- Connect to jmudb database
\c jmudb
-- -----------------------------------------------------------------------------

--
\echo
\echo Query #5
--
-- For each subject, how many sections in Fall 2024 were taught with more than
-- 15 students enrolled? Be careful not to count the same section twice!
--
-- Schema: subject, count
--  Order: count (descending), subject

SELECT subject,
       COUNT(DISTINCT nbr) AS count
FROM enrollment
WHERE term = 1248
  AND enrolled > 15
GROUP BY subject
ORDER BY count DESC, subject
LIMIT 200;

--
\echo
\echo Query #6
--
-- For each instructor, count the total number of students they taught in CS
-- courses over the past three academic years (Summer 2022 to Spring 2025).
--
-- Schema: instructor, students (calculated value)
--  Order: students (descending), instructor

SELECT instructor, SUM(students.enrolled) AS students
FROM (
    SELECT DISTINCT instructor, term, nbr, enrolled
    FROM enrollment
    WHERE subject = 'CS'
        AND term BETWEEN 1225 AND 1251
) as students
GROUP BY instructor
ORDER BY students DESC, instructor
LIMIT 200;

--
\echo
\echo Query #7
-- List all sections in Fall 2024 with more that 100 students enrolled, and
-- rank them by course (i.e., the section with the most students is rank #1).
--
-- Schema: subject, number, nbr, enrolled, rank
--  Order: subject, number, nbr

SELECT
    subject,
    number,
    nbr,
    enrolled,
    RANK() OVER (
        PARTITION BY subject, number
        ORDER BY enrolled DESC
    ) AS rank
FROM (
    SELECT subject, number, nbr, enrolled
    FROM enrollment
    WHERE term = 1248 AND enrolled > 100
    GROUP BY subject, number, nbr, enrolled
)
ORDER BY subject, number, nbr
LIMIT 200;

--
\echo
\echo Query #8
-- Rank departments in Spring 2025 by their total enrollment, i.e., who has the
-- most students enrolled across all sections. (Hint: two nested subqueries)
--
-- Schema: subject, total, rank
--  Order: subject


SELECT t1.subject,
       t1.total,
       RANK() OVER (ORDER BY t1.total DESC) AS rank
FROM (
    SELECT subject, SUM(enrolled) as total
    FROM (
       SELECT DISTINCT subject, term, nbr, enrolled
       FROM enrollment
       WHERE term = 1251
   )
    GROUP BY subject
) AS t1
ORDER BY subject
LIMIT 200;