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.

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.

num	company	pos	stop	num_1	company_1	pos_1	stop_1
1	LRT	1	134	1	LRT	7	217
1	LRT	1	134	1	LRT	11	134
1	LRT	1	134	1	LRT	10	79
...
1	LRT	2	97	1	LRT	7	217
1	LRT	2	97	1	LRT	11	134
...
2	LRT	9	31	2	LRT	11	217
2	LRT	9	31	2	LRT	1	168
...

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.

num	company	pos	stop	num_1	company_1	pos_1	stop_1
47	LRT	4	53	47	LRT	4	53
47	LRT	4	53	27	LRT	6	53
47	LRT	4	53	10	LRT	8	53
47	LRT	4	53	45	LRT	7	53
47	LRT	4	53	4	LRT	6	53
10	LRT	8	53	47	LRT	4	53
10	LRT	8	53	27	LRT	6	53
10	LRT	8	53	10	LRT	8	53
...