SQL Route Finder in Oracle - Recursion? - sql

Question

I am trying to build a simple route finder which calculates and stores the nodes of which a route traverses to get from A -- B. I have two tables; One which is made up of stages (The nodes and their 'next possible hops') and a route_stage table which should be able to store each route calculated with a unique route id.
Stage Table
STAGEID START_STATION NEXT_HOP_STATION LENGTH
---------- ------------------------------ ------------------------------ ----------
1 Penzance Plymouth 78
2 Plymouth Exeter 44.8
3 Exeter Taunton 36.6
4 Exeter Salisbury 96.6
5 Salisbury Basingstoke 38.2
6 Basingstoke Southampton 52.7
7 Southampton Poole 37
8 Poole Weymouth 31.6
9 Taunton Reading 99.5
10 Reading Basingstoke 18
11 Reading Paddington 40.9
12 Taunton Bristol 48.8
13 Bristol Bath 13
14 Bath Swindon 37.5
15 Swindon Reading 39.8
Route_Stage Table
ROUTEID STAGEID
---------- ----------
1 1
1 2
1 3
1 9
1 11
2 6
2 7
2 8
2 10
2 11
For the case of the above, the route with ID 1 Starts at Penzance and traverses, Plymouth, Exeter, Taunton, Reading and terminates at Paddington. Ideally I want to create a stored procedure that takes the entry parameters of a start and end station so the code inside will be able to calculate a suitable route.
I've had a look at recursion but got a bit lost, as I am not sure how the code should react when there are multiple potential paths from a node? How would it know which one was the correct one to go down.
Any help is greatly appreciated. Thanks!

Answer 1

For a single given starting position, this will (I think.. Sorry, typing by hand on an iPad) provide a row for each route that leaves that starting point.
SELECT
LEVEL as route_step,
t1.next_hop_station as next_station,
t1.stageid
FROM
stage t1
INNER JOIN stage t2
ON t2.start_station = t1.next_hop_station
START WITH
t1.start_station = 'your start station'
CONNECT BY
PRIOR t1.start_station = t1.next_hop_station
So, for start station Penzance:
Route_Step Next_Station StageID
1. Plymouth. 1
2. Exeter. 2
3. Taunton. 3
4. Reading. 9
5. Basingstoke. 10
6. Southampton 6
7. Poole. 7
8. Weymouth 8
5. Paddington. 11
3. Salisbury 4
4. Basingstoke. 5
5. Southampton. 6
6. Poole. 7
7. Weymouth. 8
* excuse the .'s!
Wrapping that with a join on your distinct starting stations (and removing the explicit START WITH clause so that you get routes from all stations, not just a single station) will give you what you need for your output table (although as per previous comments, I'm not sure what use that structure is to you, as you lose pertinent detail):
SELECT
First_Stage.stageid as routeid,
q.stageid
FROM
(
SELECT
LEVEL as route_step,
t1.next_hop_station as next_station,
t1.stageid
FROM
stage t1
INNER JOIN stage t2
ON t2.start_station = t1.next_hop_station
CONNECT BY
PRIOR t1.start_station = t1.next_hop_station
) q
INNER JOIN stage as first_stage
ON first_stage.stageid = q.stageid
AND q.route_step = 1