內側連接兩次返回任何結果

Question

我命名節點的兩個表格和鏈接，就像這樣：

--Links: 
----------------------------------- 
id fromx fromy tox  toy 
----------------------------------- 
a1  x1  y1  x2  y2 
a2  x2  y2  x3  y3 
a3  x2  y2  x4  y4 
a4  x1  y1  x4  y4 
a5  x1  y1  x5  y5 

--Nodes: 
id  x y 
-------------- 
1  x1 y1 
2  x2 y2 
3  x3 y3 
4  x4 y4 
5  x5 y5 

我想通過匹配fromx，弗羅米和TOX，玩具生產第三個表在打擊的節點表中的X和Y鏈接表產生一個表像這樣：

linkid fromid toid 
-------------------- 
    a1  1  2 
    a2  2  3 
    a3  2  4 
    a4  1  4 
    a5  1  5 

在努力達到那個結果，我用這個查詢使用上的節點表連接兩次以下查詢，但我沒有得到任何結果。

select links.id as linkid, 
n1.id as nodeid, fromx, fromy, tox from links 
inner join nodes n1 
inner join nodes n2 
on 
links.fromx = n1.x 
and links.fromy = n1.y 
and links.tox = n2.x 
and links.toy = n2.y 

我「很高興來創建臨時表或者這樣，如果這將有助於。

Answer 1

select 
     l.id as link_id, 
     frm.id as from_id, 
     t.id as to_id 
from 
     links l 
inner join 
     nodes frm 
     on frm.x = l.fromx 
     and frm.y = l.fromy 
inner join 
     nodes t 
     on t.x = l.tox 
     and t.y = l.toy 

SQL Fiddle