不管性能如何，我会从下面的查询A和查询B得到相同的结果吗？C和D呢？
-- A
select *
from a left join b
on <blahblah>
left join c
on <blahblan>
-- B
select *
from a left join c
on <blahblah>
left join b
on <blahblan>
-- C
select *
from a join b
on <blahblah>
join c
on <blahblan>
-- D
select *
from a join c
on <blahblah>
join b
on <blahblan>
对于INNER次加入，不，顺序无关紧要.只要将 Select 从SELECT *更改为SELECT a.*, b.*, c.*，查询将返回相同的结果.
对于(LEFT、RIGHT或FULL)OUTER连接，是的，顺序很重要，而(updated)事情要复杂得多.
首先，外部连接是不可交换的，所以a LEFT JOIN b和b LEFT JOIN a不一样
外部联接也不是关联的，因此在同时涉及(交换性和关联性)属性的示例中:
a LEFT JOIN b
ON b.ab_id = a.ab_id
LEFT JOIN c
ON c.ac_id = a.ac_id
is equivalent to:
a LEFT JOIN c
ON c.ac_id = a.ac_id
LEFT JOIN b
ON b.ab_id = a.ab_id
但是:
a LEFT JOIN b
ON b.ab_id = a.ab_id
LEFT JOIN c
ON c.ac_id = a.ac_id
AND c.bc_id = b.bc_id
is not equivalent to:
a LEFT JOIN c
ON c.ac_id = a.ac_id
LEFT JOIN b
ON b.ab_id = a.ab_id
AND b.bc_id = c.bc_id
另一个(希望更简单)关联性示例.把这想象成(a LEFT JOIN b) LEFT JOIN c:
a LEFT JOIN b
ON b.ab_id = a.ab_id -- AB condition
LEFT JOIN c
ON c.bc_id = b.bc_id -- BC condition
这是is equivalent到a LEFT JOIN (b LEFT JOIN c):
a LEFT JOIN
b LEFT JOIN c
ON c.bc_id = b.bc_id -- BC condition
ON b.ab_id = a.ab_id -- AB condition
只是因为我们有"不错"的ON个条件.ON b.ab_id = a.ab_id和c.bc_id = b.bc_id都是相等判断，不涉及NULL比较.
您甚至可以使用其他运算符或更复杂的运算符，例如:ON a.x <= b.x或ON a.x = 7或ON a.x LIKE b.x或ON (a.x, a.y) = (b.x, b.y)，这两个查询仍然是等效的.
然而，如果其中任何一个查询涉及IS NULL或与空值(如COALESCE())相关的函数，例如，如果条件是b.ab_id IS NULL，那么这两个查询将不等价.