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我有一个使用远程数据库的数据库链接连接的 Oracle 数据库。我的远程数据库与 OUTER JOIN 无关，这就是异构服务将我的查询转换为几个简单查询并连接结果的原因。

例如，我有 3 个表：

create table join_email
(
EMAIL_ID FLOAT(10)
SENDER VARCHAR2(128)
)
INSERT INTO join_email VALUES (1,'bmdrrfh@gmail.com')
INSERT INTO join_email VALUES (2,'n3qcd@gmail.com')

create table join_email_receivers
(
EMAIL_ID FLOAT(10)
RECEIVER VARCHAR2(128)
)
INSERT INTO join_email_receivers VALUES (1,'9wtcptyzn@yahoo.com')
INSERT INTO join_email_receivers VALUES (1,'8w7o5@yahoo.com')
INSERT INTO join_email_receivers VALUES (1,'jlwtc@yahoo.com')
INSERT INTO join_email_receivers VALUES (2,'fpm@yandex.ru')
INSERT INTO join_email_receivers VALUES (2,'a@mail.com')

create table join_email_cc
(
EMAIL_ID FLOAT(10)
CC VARCHAR2(128)
)
INSERT INTO join_email_cc VALUES (1,'rg1yzjc@mail.com')

我想从 1 表中查询行，并通过 email_id 从第 2 和第 3 左连接行。我的查询看起来像：

select em.sender, emr.receiver, emcc.cc
from join_email@DG4 em
LEFT JOIN join_EMAIL_RECEIVERS@DG4 emr on emr.email_id=em.email_id
LEFT JOIN join_EMAIL_CC@DG4 emcc on emcc.email_id=em.email_id
where em.sender = 'bmdrrfh@gmail.com' and emr.receiver = '9wtcptyzn@yahoo.com';

问题是异构服务将此查询转换为以下两个查询：

<SELECT A2."EMAIL_ID",A2."SENDER",A1."RECEIVER" FROM "JOIN_EMAIL" A2,"JOIN_EMAIL_RECEIVERS" A1 WHERE A1."EMAIL_ID"=A2."EMAIL_ID" AND A2."SENDER"='bmdrrfh@gmail.com' AND A1."RECEIVER"='9wtcptyzn@yahoo.com'>

和

<SELECT "CC","EMAIL_ID" FROM "JOIN_EMAIL_CC">

第二个查询是错误的全扫描查询，它应该是 email_id 的 WHERE 子句。

我的问题是，如何告诉异构服务如何以正确的顺序转换我的查询？

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1 回答 1

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第一个LEFT OUTER JOIN实际上是INNER JOIN由于列上的相等谓词emr.receiver，因此它被Oracle正确转换。

如果远程站点上运行的 RDBMS 本身支持外连接，您可以在远程站点上创建一个视图，省略该WHERE子句，例如

在远程站点（DG4在您的示例中）：

create view my_view as
select em.sender, emr.receiver, emcc.cc
from join_email em
LEFT JOIN join_EMAIL_RECEIVERS emr on emr.email_id=em.email_id
LEFT JOIN join_EMAIL_CC emcc on emcc.email_id=em.email_id

然后在本地站点上：

select *
  from my_view@DG4
 where em.sender = 'bmdrrfh@gmail.com'
   and emr.receiver = '9wtcptyzn@yahoo.com'
于 2014-10-29T14:44:03.827 回答