From: Ryan <rgaffuri@cox.net>
To: pgsql-sql@postgresql.org
Subject: Re: double linked list
Date: Wed, 29 Jan 2003 21:30:11 GMT
Message-ID: <DJXZ9.53859$GX4.2201155@news2.east.cox.net> (raw)
References: <2ee07259.0301280744.582e4b7c@posting.google.com>
<c0d87ec0.0301281554.581797ee@posting.google.com>
<2ee07259.0301290412.5eb3290a@posting.google.com>
<c0d87ec0.0301291315.7ae946eb@posting.google.com>
are you joe celko, guy who wrote those sql books?
"--CELKO--" <71062.1056@compuserve.com> wrote in message
news:c0d87ec0.0301291315.7ae946eb@posting.google.com...
> >> The table at hand is more a kind of a collection of graphs where I
> want to find all possible paths between a given starting point and a
> given end point. <<
>
> For the reachabiity index of a general graph, you need Warshal's
> algorithm.
>
> Let V = number of nodes in the graph
> Let A[i,j] be the adjacency matrix for the undirected graph
>
> FOR j:= 1 TO V
> DO FOR i:= 1 TO V
> DO IF A[i,j] = 1
> THEN FOR k := 1 TO V
> DO IF A[j,k]] = 1
> THEN A[i,k]] := 1;
>
> You can also do a summation to get the length of the path from i to j.
> You can concatenate names of the nodes into a string that gives the
> path, etc.
>
> Her is a first attempt at some SQL; I am sure it can be done better
>
> CREATE TABLE Graph
> (i CHAR(2) NOT NULL,
> j CHAR(2) NOT NULL,
> flag CHAR(1) NOT NULL DEFAULT 'n'
> CHECK (flag IN ('n', 'y')),
> PRIMARY KEY (i,j));
>
> INSERT INTO Graph (i, j, flag)
> SELECT DISTINCT G1.i, G2.j, 'y'
> FROM Graph AS G1, Graph AS G1
> WHERE G1.i <> G2.j
> AND EXISTS
> (SELECT *
> FROM Graph AS G3
> WHERE G3.i = G1.j
> AND G3.j = G2.i)
> AND NOT EXISTS
> (SELECT *
> FROM Graph AS G3
> WHERE (G3.i = G1.i AND G3.j = G1.j))
> OR (G3.i = G2.i AND G3.j = G2.j));
>
> You wll have to run this statement until the size of Graph does not
> change -- no new rows are being added.
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To: pgsql-sql@postgresql.org
Cc: rgaffuri@cox.net
Subject: Re: double linked list
In-Reply-To: <DJXZ9.53859$GX4.2201155@news2.east.cox.net>
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