Undirected graph in Oracle SQL - sql

Question

I'm trying to represent undirected graph in Oracle SQL, for example, I have stations graph:
CREATE TABLE station (
station_id INTEGER NOT NULL
);
CREATE TABLE station_link (
from_station INTEGER NOT NULL,
to_station INTEGER NOT NULL
);
This is obviously directed graph, but I have no idea, how to make it undirected.
Point: I need to get all vertices, which have path to current vertex and information about their level (how many vertices on this path).
For directed graph it is pretty easy:
SELECT sl.to_station, LEVEL
FROM station_link sl
START WITH sl.from_station = :curVertex
CONNECT BY NOCYCLE PRIOR sl.to_station = sl.from_station
But so we will get only one-way verticies.
Question: Do this problem have solution, except adding additional links (2 -> 1 for 1 -> 2)?
There is sql fiddle for tests: http://sqlfiddle.com/#!4/6c09e/24

Answer 1

For a "fast win" you can use your structure "as is", but for every edge you shold have two records in station_link table.
If you want "not_so_fast_but_without_doble_edge_records_please win", you can use this weirdo-trick:
SELECT
x.TO_STATION,
x.LVL
FROM (
SELECT sl.to_station, LEVEL as lvl
FROM station_link sl
START WITH sl.from_station = :curVertex
CONNECT BY NOCYCLE PRIOR sl.to_station = sl.from_station
UNION ALL
SELECT sl.from_station as to_station , LEVEL as lvl
FROM station_link sl
START WITH sl.to_station = :curVertex
CONNECT BY NOCYCLE PRIOR sl.from_station = sl.to_station
) x
It will do the work. Actually, it just combines two traversal directions.
But if I'll want to implement some serious algorithms on graphs in PLSQL, I would look to SDO_GEOMETRY data type and Oracle Spatial And Graphs Datasheets.