Re: double linked list

>> The table at hand is more a kind of a collection of graphs where I
want to find all possible paths between a given starting point and a
given end point. <<

For the reachabiity index of a general graph, you need Warshal's
algorithm.

Let V = number of nodes in the graph
Let A[i,j] be the adjacency matrix for the undirected graph

FOR j:= 1 TO VDO FOR i:= 1 TO V    DO IF A[i,j] = 1       THEN FOR k := 1 TO V             DO IF A[j,k]] = 1
   THEN A[i,k]] := 1;
 

You can also do a summation to get the length of the path from i to j.
You can concatenate names of the nodes into a string that gives the
path, etc.

Her is a first attempt at some SQL; I am sure it can be done better

CREATE TABLE Graph 
(i CHAR(2) NOT NULL,j CHAR(2) NOT NULL,flag CHAR(1) NOT NULL DEFAULT 'n'  CHECK (flag IN ('n', 'y')),PRIMARY KEY
(i,j));

INSERT INTO Graph (i, j, flag)SELECT DISTINCT G1.i, G2.j, 'y'  FROM Graph AS G1, Graph AS G1 WHERE G1.i <> G2.j   AND
EXISTS      (SELECT *          FROM Graph AS G3         WHERE G3.i = G1.j           AND G3.j = G2.i)   AND NOT EXISTS
   (SELECT *          FROM Graph AS G3         WHERE (G3.i = G1.i AND G3.j = G1.j))            OR (G3.i = G2.i AND G3.j
=G2.j));
 

You wll have to run this statement until the size of Graph does not
change -- no new rows are being added.