tutorial9.sql

-- Self join

-- 1. How many stops are in the database.

SELECT COUNT(*)
FROM stops;

-- 2. Find the id value for the stop 'Craiglockhart'

SELECT id
FROM stops
WHERE name = 'Craiglockhart';

-- 3. Give the id and the name for the stops on the '4' 'LRT' service.

SELECT id, name
FROM stops
JOIN route
ON id = stop
WHERE num = '4' AND company = 'LRT';

-- 4. Add a HAVING clause to restrict the output to these two routes.

SELECT company, num, COUNT(*)
FROM route
WHERE stop = 149 OR stop = 53
GROUP BY company, num
HAVING COUNT(*) >= 2;

-- 5. Change the query so that it shows the services from Craiglockhart to London Road.

SELECT a.company, a.num, a.stop, b.stop
FROM route a JOIN route b
ON a.company = b.company AND a.num = b.num
WHERE a.stop = 53 AND b.stop = 149;

-- 6. Change the query so that the services between 'Craiglockhart' and 'London Road' are shown.

SELECT a.company, a.num, stopa.name, stopb.name
FROM route a 
JOIN route b 
ON a.company = b.company AND a.num = b.num
JOIN stops stopa ON a.stop = stopa.id
JOIN stops stopb ON b.stop = stopb.id
WHERE stopa.name = 'Craiglockhart'
  AND stopb.name = 'London Road';

-- 7. Give a list of all the services which connect stops 115 and 137 ('Haymarket' and 'Leith')

SELECT DISTINCT a.company, a.num
FROM route a
JOIN route b
ON a.company = b.company AND a.num = b.num
WHERE a.stop = 115 AND b.stop = 137;

-- 8. Give a list of the services which connect the stops 'Craiglockhart' and 'Tollcross'

SELECT a.company, a.num
FROM route a 
JOIN route b 
ON a.company = b.company AND a.num = b.num
JOIN stops stopa ON a.stop = stopa.id
JOIN stops stopb ON b.stop = stopb.id
WHERE stopa.name = 'Craiglockhart'
  AND stopb.name = 'Tollcross';

/* 9. Give a distinct list of the stops which may be reached from 'Craiglockhart' by taking one bus,
including 'Craiglockhart' itself, offered by the LRT company. Include the company and
bus no. of the relevant services. */

SELECT DISTINCT stopb.name, a.company, a.num
FROM route a 
JOIN route b 
ON a.company = b.company AND a.num = b.num
JOIN stops stopa ON a.stop = stopa.id
JOIN stops stopb ON b.stop = stopb.id
WHERE stopa.name = 'Craiglockhart';