(A连接B)连接(C连接D)

Question

我想确定加入的具体顺序。

SELECT *
FROM (lives_in as t1 NATURAL JOIN preferences p1) l1
JOIN (lives_in t2 NATURAL JOIN preferences p2) l2
ON l1.dormid = l2.dormid

返回一个错误。

有人能帮忙吗？非常感谢!

Answer 1

您的别名查询缺少一个SELECT子句，因此尝试如下：

SELECT *
FROM (
    select * -- added this
    FROM lives_in as t1
    NATURAL JOIN preferences p1) l1
JOIN (
    select * -- added this
    FROM lives_in t2
    NATURAL JOIN preferences p2) l2
ON l1.dormid = l2.dormid

Answer 2

连接的顺序与结果无关。你可能想要这样的东西：

SELECT *
FROM lives_in t1 
NATURAL JOIN preferences p1 ON p1.some_id = t1.id
NATURAL JOIN preferences p2 ON p2.some_id = t1.id

而且，大多数人称它为内部连接，而不是自然连接，顺便说一下。

Answer 3

希望能帮上忙。

SELECT *
FROM (
select * from lives_in as t1 NATURAL JOIN preferences p1
) l1
JOIN (
select * from lives_in t2 NATURAL JOIN preferences p2
) l2
ON l1.dormid = l2.dormid