SQLZOO 09

Self join

1. How many stops are in the database.

SELECT COUNT(name)
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, route 
WHERE num = '4' 
AND company = 'LRT' 
AND id = stop;

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 = 
(SELECT id FROM stops WHERE name = 'Craiglockhart') 
AND b.stop = 
(SELECT id FROM stops WHERE name = 'London Road');

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';

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, route b 
WHERE a.num = b.num 
AND a.company = b.company 
AND 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.
   By taking the bus of the LRT company (including the'Craiglockhart' itself), provide the stations that can be reached from "Craiglockhart", list the names of these stations, the company and the bus route number of these routes

SELECT name,a.company, a.num 
FROM route a, route b, stops s 
WHERE a.num = b.num 
AND a.stop = s.id
AND a.company = b.company
AND a.company = 'LRT' 
AND b.stop =
(SELECT id FROM stops WHERE name = 'Craiglockhart');

SELECT stopa.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 stopb.name='Craiglockhart' 
AND a.company = 'LRT' ;

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.
    How to transfer 2 cars from Craiglockhart to Lochend, list the first car's car number, company, transfer station, and the second car's car number, company

SELECT DISTINCT S1.num, S1.company, stops.name, S2.num, S2.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')
)S1
  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')
)S2
ON (S1.stop = S2.stop)
JOIN stops ON(stops.id = S1.stop);