9 Self JOIN.sql

-- Self JOIN --
-- URL: https://sqlzoo.net/wiki/Self_join

-- 1.
-- How many stops are in the database.
SELECT Count(id)
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 stop = id
WHERE  num = '4';

-- Routes and stops

-- 4.
-- The query shown gives the number of routes that visit either London Road (149) or Craiglockhart (53). Run the query and notice the two services that link these stops have a count of 2. 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.
-- 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.
-- The query shown is similar to the previous one, however by joining two copies of the stops table we can refer to stops by name rather than by number. Change the query so that the services between 'Craiglockhart' and 'London Road' are shown. If you are tired of these places try 'Fairmilehead' against 'Tollcross'
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';

-- Using a self join

-- 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 stopsb.name,
       a.company,
       a.num
FROM   route a
       JOIN route b
         ON ( a.company = b.company
              AND a.num = b.num )
       JOIN stops stopsa
         ON a.stop = stopsa.id
       JOIN stops stopsb
         ON b.stop = stopsb.id
WHERE  stopsa.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 a.num,
       a.company,
       bstop.name,
       c.num,
       c.company
FROM   route AS a
       JOIN route AS b
         ON ( a.company = b.company
              AND a.num = b.num )
       JOIN route AS c
         ON ( b.stop = c.stop )
       JOIN route AS d
         ON ( c.company = d.company
              AND c.num = d.num )
       JOIN stops AS astop
         ON ( a.stop = astop.id )
       JOIN stops AS bstop
         ON ( b.stop = bstop.id )
       JOIN stops AS cstop
         ON ( c.stop = cstop.id )
       JOIN stops AS dstop
         ON ( d.stop = dstop.id )
WHERE  astop.name = 'Craiglockhart'
       AND dstop.name = 'Lochend'
       AND bstop.id = cstop.id
ORDER  BY CONVERT(a.num, CHAR),
          bstop.name,
          CONVERT(c.num, CHAR);