Re: double linked list

are you  joe celko, guy who wrote those sql books?

"--CELKO--" <71062.1056@compuserve.com> wrote in message
news:c0d87ec0.0301291315.7ae946eb@posting.google.com...
> >> 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 V
>  DO 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.