为什么子查询在DB2的ON子句中不起作用

Question

为什么这个简单的查询在oracle中可以正常工作，而在DB2中却不起作用：

select * 
from 
sysibm.dual d1 
left join sysibm.dual d2 on 1=1 and exists (select 1 from sysibm.dual)

将涉及子查询的条件移到where子句可能会有所帮助，但这将限制外部联接进入内部。

Answer 1

当我尝试运行查询时，出现-338错误 ，根据信息中心（请参阅链接），对ON子句有以下限制：

由于以下原因之一，与JOIN运算符或MERGE语句关联的ON子句无效。

 * The ON clause cannot include any subqueries. * Column references in an ON clause must only reference columns of tables that are in the scope of the ON clause. * Scalar fullselects are not allowed in the expressions of an ON clause. * A function referenced in an ON clause of a full outer join must be deterministic and have no external action. * A dereference operation (->) cannot be used. * A SQL function or SQL method cannot be used. * The ON clause cannot include an XMLQUERY or XMLEXISTS expression. 

我不确定查询是否可行，但您认为也许可以重新编写如下代码：

select * 
from 
sysibm.dual d1 
left join (
    SELECT dl.*,
    CASE WHEN EXISTS (SELECT 1 FROM sysibm.dual)
         THEN 1
         ELSE 0
    END AS jn
    FROM sysibm.dual dl
) D2
  on 1=1 and 1=d2.jn

Answer 2

这在DB2 V10.1中有效！ 没有安装修订包。