数据表：stops-route

stops:   id,name

route:   num,company,pos,stop

Q1

How many stops are in the database.

SELECT COUNT(id) FROM stops

Q2

Find the id value for the stop 'Craiglockhart'

SELECT id FROM stops WHERE name='Craiglockhart'

Q3

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'

Q4

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

Q5

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')

SELF JOIN

针对相同的表进行的连接

SELECT 列名
FROM 表名 AS 别名1
JOIN 表名 AS 别名2
ON 连接条件
 

1，在同一张表内进行比较

2，找出列的组合

3，查找部分内容重复的记录

Q6

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.nameFROM 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

Q7

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

Q8

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

SELECT DISTINCT a.company,a.num FROM route a
JOIN route b ON (a.company=b.company AND a.num=b.num)
JOIN stops x ON (x.id=a.stop)
JOIN stops y ON (y.id=b.stop)WHERE x.name='Craiglockhart'
AND y.name='Tollcross'

Q9

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 y.name,a.company,a.num FROM route a
JOIN route b ON (a.company=b.company AND a.num=b.num)
JOIN stops x ON (x.id=a.stop)
JOIN stops y ON (y.id=b.stop)WHERE x.name='Craiglockhart'

Q10

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,name,b.num,b.company FROM (SELECT x.num,x.company,y.stop FROM route x JOIN route y ON (x.num=y.num AND x.company=y.company AND x.stop<>y.stop) WHERE x.stop=(select id from stops where name ='Craiglockhart')) AS aJOIN (SELECT q.num,q.company,q.stop FROM route q JOIN route w ON (q.num=w.num AND q.company=w.company AND q.stop<>w.stop) WHERE w.stop=(select id from stops where name ='Lochend')) AS b
ON a.stop=b.stopJOIN stops ON a.stop=stops.idORDER BY a.num,stops.name,b.num