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SQLZoo Solutions: Self JOIN

This document contains my solutions to the SQLZoo 'Self JOIN' section using MySQL syntax, along with my personal learning notes and explanations.

Problem 1

How many stops are in the database.

My Solution:

SELECT COUNT(id)
FROM stops;

Problem 2

Find the id value for the stop 'Craiglockhart'.

My Solution:

SELECT id
FROM stops
WHERE name = 'Craiglockhart';

Problem 3

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

My Solution:

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

Problem 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.

My Solution:

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

Problem 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.

My Solution:

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;

Problem 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'.

My Solution:

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';

Problem 7

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

My Solution:

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

Problem 8

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

My Solution:

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';

Problem 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.

My Solution:

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'
  AND (b.pos >= a.pos OR a.pos >= b.pos);

Problem 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.

My Solution:

SELECT 
  bus1.num, 
  bus1.company,      
  stops.name,        
  bus2.num,          
  bus2.company       
FROM (
  SELECT start1.num, start1.company, stop1.stop
  FROM route AS start1
    JOIN route AS stop1 ON start1.num = stop1.num
      AND start1.company = stop1.company
      AND start1.stop != stop1.stop
  WHERE start1.stop = (SELECT id FROM stops WHERE name = 'Craiglockhart')
) AS bus1
JOIN (
  SELECT start2.num, start2.company, start2.stop
  FROM route AS start2
    JOIN route AS stop2 ON start2.num = stop2.num
      AND start2.company = stop2.company
      AND start2.stop != stop2.stop
  WHERE stop2.stop = (SELECT id FROM stops WHERE name = 'Lochend')
) AS bus2
  ON bus1.stop = bus2.stop
JOIN stops ON bus1.stop = stops.id
ORDER BY bus1.num, bus1.company, stops.name, bus2.num, bus2.company;

My Notes:

  • Use two subqueries for bus routes from Craiglockhart to a transfer stop, and from transfer to Lochend.
  • JOIN on matching transfer stop IDs.