9_self_join.md

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. 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';