Question

I have a sqlite table representing a weighted directed graph. The columns are as follows:

node1 | node2 | weight(node1,node2)

Instead I need this table:

node1 | node2 | weight(node1,node2) | weight(node2,node1)

(node2 | node1 | weight(node2,node1) | weight (node1, node2) should not exist in the table)

I am not familiar with SQL that well, so I couldn’t figure out how to make the new table from the original table in sqlite3.

Thanks for your help.

Answer 1

I assume your node1 and node2 are integer foreign keys linking to a node table, and the table you mention is an edge table?

Assuming the edge table has been created with something like:

CREATE TABLE edges( node1 INTEGER, node2 INTEGER, weight REAL );

How about something like (assuming no self-arcs and for every link from a->b there is also a link from b->a):

CREATE TABLE newedges( node1 INTEGER, node2 INTEGER, weight1 REAL, weight2 REAL );

INSERT INTO newedges
    SELECT e1.node1, e1.node2, e1.weight, e2.weight
    FROM edges AS e1 INNER JOIN edges AS e2
    ON e1.node1=e2.node2 AND e1.node2=e2.node1
    WHERE e1.node1 < e1.node2;

The self-join collates forward and backwards edges, and the requirement that e1.node1 is less than e1.node2 ensures that you only see each collated edge once.

Edit in response to a request to fill in zeros for missing backwards edge:

For missing backwards edges, you can do a LEFT JOIN and use a CASE statement to fill in the gaps with zeros:

INSERT INTO newedges
    SELECT
        e1.node1,
        e1.node2,
        e1.weight,
        CASE WHEN e2.weight IS NULL THEN 0.0 ELSE e2.weight END
    FROM edges AS e1 LEFT JOIN edges AS e2
    ON e1.node1=e2.node2 AND e1.node2=e2.node1
    WHERE e1.node1 < e1.node2;

Hope that helps!