Using a self join
Edinburgh Buses: self join
A table may be joined with itself. When this happens we need some mechanism for distinguishing the instances of the table. Labels may be introduced in the FROM clause - a dot is used to separate the label and attribute name when used elsewhere in the statement.
Examples of self joins on the route table
Self join of route on (num, company)
We might join the route table on the (num, company) pair. The result is a list of all pairs of stops which share a service.
SELECT * FROM route R1, route R2 WHERE R1.num=R2.num AND R1.company=R2.company
This is a large table with over 11000 entries. An extract is shown below. There are 121 entries for the LRT 1 service alone. There are 11 stops on the number 1. It is a circular route - the first and last stops are identical.
Self join of route on stop
We might join the route table with itself on the stop field. The result is a list of all pairs of services which share a stop.
SELECT * FROM route R1, route R2 WHERE R1.stop=R2.stop;
In the extract below some of the entries for stop 53, 'Craiglockhart' are shown.