9_self_JOIN.sql

-- 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 route.stop = stops.id
  WHERE num = 4
  ORDER BY pos;

-- 4. The query shown gives the number of routes that visit either London Road (149) or Craiglockhart (53). 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(*) > 1;

-- 5. Execute the self join shown and observe that b.stop gives all the places you can get to from Craiglockhart, without changing routes. 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 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.id = 115 AND stopb.id = 137
  GROUP BY a.company, a.num;

-- 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'
  GROUP BY a.company, a.num;

-- 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 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';

-- 10. Find the routes involving two buses that can go from Craiglockhart to Lochend. Show the bus no. and company for the first bus, the name of the stop for the transfer, and the bus no. and company for the second bus.

SELECT DISTINCT Depart.num, Depart.company, stops.name, Arrive.num, Arrive.company 
  FROM 
      (SELECT a.company, a.num, b.stop FROM route a
        JOIN route b ON (a.company=b.company AND a.num=b.num)
        WHERE a.stop = 
            (SELECT id FROM stops
              WHERE name='Craiglockhart')
      )Depart
  JOIN 
      (SELECT a.company, a.num, b.stop FROM route a
        JOIN route b ON (a.company=b.company AND a.num=b.num)
        WHERE a.stop = 
            (SELECT id FROM stops
              WHERE name='Lochend')
      )Arrive
  ON (Arrive.stop = Depart.stop)
  JOIN stops ON (stops.id = Depart.stop)
  ORDER BY Depart.num, stops.name, Arrive.num, Arrive.num;