这是一个困扰我好几天的问题,我搜索并搜索但找不到任何令人信服的答案!
简单的问题,为什么在 SQL 中限制有 2 个外部联接,即使在使用不同列的同一个表上,请检查下面的查询以更好地理解。我也可以使用嵌套子查询或 ANSI 连接来克服它们,但是为什么它甚至首先使用 (+) 运算符受到限制!
在这个问题中,我指的是错误:
ORA-01417: a table may be outer joined to at most one other table
我想问的是为什么允许这样做:
select * from
a, b, c
where a.a1 = b.b1
and a.a2 = c.c1
以及为什么不允许这样做:
select * from
a, b, c
where a.a1(+) = b.b1
and a.a2(+) = c.c1
请不要管 ANSI 和嵌套子查询
最佳答案
Oracle 文档中描述了该限制:Outer Joins
Oracle recommends that you use the FROM clause OUTER JOIN syntax rather than the Oracle join operator. Outer join queries that use the Oracle join operator (+) are subject to the following rules and restrictions, which do not apply to the FROM clause OUTER JOIN syntax:
...
In a query that performs outer joins of more than two pairs of tables, a single table can be the null-generated table for only one other table. For this reason, you cannot apply the (+) operator to columns of B in the join condition for A and B and the join condition for B and C. Refer to SELECT for the syntax for an outer join.
这基本上意味着(在 ANSI/ISO 语法中描述)您不能使用旧的
(+)
在 ANSI/ISO 中完全有效的语法:--- Query 1 ---
a
RIGHT JOIN b
ON a.x = b.x
RIGHT JOIN c
ON a.y = c.y
或者:--- Query 1b ---
c
LEFT JOIN
b LEFT JOIN a
ON a.x = b.x
ON a.y = c.y
这只是旧 Oracle 语法的众多限制之一。至于这种限制的原因,可能是实现细节或/和这种连接的歧义。虽然上面的两个连接是 100% 等价的,但以下不等价于上面两个:
--- Query 2 ---
a
RIGHT JOIN c
ON a.y = c.y
RIGHT JOIN b
ON a.x = b.x
见中的测试SQL-Fiddle . 所以问题来了。专有连接应该如何解释为查询 1 或 2?FROM a, b, c
WHERE a.y (+) = c.y
AND a.x (+) = b.x
如果表出现在(2 个或更多)外部联接的左侧,则没有限制。即使使用旧语法,这些也是完全有效的:
FROM a
LEFT JOIN b ON a.x = b.x
LEFT JOIN c ON a.y = c.y
...
LEFT JOIN z ON a.q = z.q
FROM a, b, ..., z
WHERE a.x = b.x (+)
AND a.y = c.y (+)
...
AND a.q = z.q (+)
关于sql - 同一个表上的 2 个外部联接?,我们在Stack Overflow上找到一个类似的问题: https://stackoverflow.com/questions/17117096/