SQL Postgres display details of edges - sql

Question

I have a table of nodes say
Table Node
Column| type
------|---------------
id | int
x | int
y | int
z | text
for example,
id | x | y | z
---|---|---|---
0 | 0 | 0 |'a'
1 | 0 | 1 |'b'
2 | 1 | 0 |'c'
3 | 1 | 1 |'d'
and
Table Edge
Column| type
------|---------------
source| int references Node.id
target| int references Node.id
for example,
source | target
-------|-------
0 | 1
1 | 3
3 | 2
assuming a node doesn't have an edge to itself, and every source is unique
I want to display the result of the entire edge information
source.x | source.y | target.x | target.y
---------|----------|----------|---------
0 | 0 | 0 | 1
0 | 1 | 1 | 1
1 | 1 | 1 | 0
I have tried many joins(self join of node with inner join with edge), (left join of edge with result of self join of node)
How can I achieve this selection result?

Answer 1

I think you want two joins:
select ns.x, ns.y, nt.x, nt.y
from edge e join
nodes n1
on e.source = ns.id join
nodes nd
on e.target = nt.id

Answer 2

In case of PostgreSQL, you can play with the RECURSIVE WITH clause.
a) build a path:
CREATE LOCAL TEMPORARY TABLE
node(id,x,y,z) AS (
SELECT 0,0,0,'a'
UNION ALL SELECT 1,0,1,'b'
UNION ALL SELECT 2,1,0,'c'
UNION ALL SELECT 3,1,1,'d'
)
;
CREATE LOCAL TEMPORARY TABLE
edge(source,target) AS (
SELECT 0,1
UNION ALL SELECT 1,3
UNION ALL SELECT 3,2
)
;
WITH RECURSIVE backbone AS (
SELECT
source::CHAR(1)||','||target::CHAR(1) AS path
, 1 AS hops
, *
FROM edge
UNION ALL SELECT
p.path||','||c.target::CHAR(1) AS path
, p.hops + 1 AS hops
, c.*
FROM backbone p JOIN edge c ON c.source=p.target
)
SELECT * FROM backbone;
path | hops | source | target
---------+------+--------+--------
0,1 | 1 | 0 | 1
1,3 | 1 | 1 | 3
3,2 | 1 | 3 | 2
0,1,3 | 2 | 1 | 3
1,3,2 | 2 | 3 | 2
0,1,3,2 | 3 | 3 | 2
b) for the paths with 2 and 3 hops, get the first and last node id from the path:
-- as above, but instead of the last line: "SELECT * FROM backbone" ...
,
origin_and_dest AS (
SELECT
*
, SPLIT_PART(path,',',1 )::INT AS origin
, SPLIT_PART(path,',',hops + 1)::INT AS destination
FROM backbone
WHERE hops >=2
)
SELECT * FROM origin_and_dest;
path | hops | source | target | origin | destination
---------+------+--------+--------+--------+-------------
0,1,3 | 2 | 1 | 3 | 0 | 3
1,3,2 | 2 | 3 | 2 | 1 | 2
0,1,3,2 | 3 | 3 | 2 | 0 | 2
With the ultimate source id and the ultimate target id out of the path, join back to nodes:
-- same as above, but instead of the last line: "SELECT * FROM origin_and_dest" ...
SELECT
origin.x AS source_x
, origin.y AS source_y
, dest.x AS target_y
, dest.y AS target_y
FROM origin_and_dest
JOIN node AS origin ON origin=origin.id
JOIN node AS dest ON destination=dest.id
;
source_x | source_y | target_y | target_y
----------+----------+----------+----------
0 | 0 | 1 | 0
0 | 0 | 1 | 1
0 | 1 | 1 | 0
Not quite your result, but then I don't really know what you mean by "entire edge information" ... can you be more precise?