選擇傳遞關係的

Question

我有一個表（在MySQL）這樣的：

subject | predicate | object 
A  | P1  | X 
X  | P2  | B 

我想通過使用此表就知道傳遞關係，所以結果應該是這樣的：

element1 | predicate1 | pivot | predicate2 | element2 
A  | P1   | X  | P2   | B 

我已經嘗試了通過使用嵌套查詢構造查詢，但是最後我有語法錯誤（我認爲p2（在嵌套查詢）不能在主查詢來確定）

select p1.subject, p1.predicate, p1.object, p2.predicate, p2.object 
from some_relation p1 
where p1.subject = 'A' 
and p1.object = (select p2.subject from some_relation p2 where p2.object = 'B'); 

任何人都知道如何做這種查詢？可能嗎？

Answer 1

使用self join：

SELECT 
a.subject as element1, 
a.predicate as predicate2, 
a.object as pivot, 
b.predicate as predicate2, 
b.subject as element2 FROM 
table a JOIN 
table b ON a.object = b.subject; 

Answer 2

我會寫一個簡單的JOIN：

select p1.subject, p1.predicate, p1.object, p2.predicate, p2.object 
from some_relation p1 
inner join some_relation p2 on p1.object = p2.subject and p2.object = 'B' 
where p1.subject = 'A' 

還是你的意思是不同的東西？

Answer 3

你不得不加入表本身：

select p1.subject, p1.predicate, p1.object, p2.predicate, p2.object 
from some_relation p1 inner join some_relation p1 
    on p1.object = p2.subject 
where p1.subject = 'A'; 

請注意，你不能這樣做下去。例如，上述查詢只能返回一個「跳」。如果你有一個傳遞鏈，在普通的SQL中很難做到。