Question

为什么这个简单的查询在oracle中工作正常但在DB2中不起作用：

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

将子查询涉及条件移动到where子句可能会有所帮助，但这会将外连接限制在内部。

Answer 1

当我尝试运行您的查询时，我得到-338 error，根据信息中心（请参阅链接），ON子句有以下限制：

与JOIN运算符或MERGE语句关联的ON子句由于以下原因之一而无效。

* 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.

我不确定你的查询是否可行，但你认为也许你可以重写这样的东西：

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

这适用于DB2 V10.1！没有安装fixpack。

为什么子查询在DB2中的ON子句中不起作用

2 个答案: