Question

我无法理解Oracle返回这个奇怪结果的原因。我认为代码非常清楚。

我预料到了无条件=（在OUTER JOIN cond中为非null）+（在OUTER JOIN cond中为null）

因为我将外连接右表列中的IS NULL / IS NOT NULL解释为EXISTS / NOT EXISTS条件。

为什么我错了？

DESCRIPTION                          COUNT(1)
---------------------------------- ----------
No condition                             6403
is NOT null in OUTER JOIN cond           6403
is not null in where cond                6401
is null in OUTER JOIN cond               6247
is null in where cond                       2
proof flh_id_messaggio is not null          0
proof flh_stato is not null                 0


  SELECT 'is null in OUTER JOIN cond ' description, count(1)
    FROM    netatemp.TMP_BACKLOG_NOBILLING2013 t
         LEFT OUTER JOIN
            eni_flussi_hub c ON
            c.flh_id_messaggio = t.flh_id_messaggio  
            AND c.flh_stato is null   
   WHERE 1 = 1 
       And t.flh_stato = 'PA'
         AND t.OWNER = 'ETL' 
UNION
  SELECT 'is NOT null in OUTER JOIN cond ' description, count(1)
    FROM    netatemp.TMP_BACKLOG_NOBILLING2013 t
         LEFT OUTER JOIN
            eni_flussi_hub c ON
            c.flh_id_messaggio = t.flh_id_messaggio  
            AND c.flh_stato is not null   
   WHERE 1 = 1 
       And t.flh_stato = 'PA'
         AND t.OWNER = 'ETL'
UNION
  SELECT 'is null in where cond ' description, count(1)
    FROM    netatemp.TMP_BACKLOG_NOBILLING2013 t
         LEFT OUTER JOIN
            eni_flussi_hub c ON
            c.flh_id_messaggio = t.flh_id_messaggio                
   WHERE 1 = 1 
       And t.flh_stato = 'PA'
         AND t.OWNER = 'ETL'
         AND c.flh_stato is null 
UNION
  SELECT 'is not null in where cond ' description, count(1)
    FROM    netatemp.TMP_BACKLOG_NOBILLING2013 t
         LEFT OUTER JOIN
            eni_flussi_hub c ON
            c.flh_id_messaggio = t.flh_id_messaggio                
   WHERE 1 = 1 
       And t.flh_stato = 'PA'
         AND t.OWNER = 'ETL'
         AND c.flh_stato is not null 
UNION
  SELECT 'No condition' description, count(1)
    FROM    netatemp.TMP_BACKLOG_NOBILLING2013 t
         LEFT OUTER JOIN
            eni_flussi_hub c ON
            c.flh_id_messaggio = t.flh_id_messaggio                
   WHERE 1 = 1 
       And t.flh_stato = 'PA'
         AND t.OWNER = 'ETL'
UNION select 'proof flh_stato is not null' description, count(1)
from eni_flussi_hub
where flh_stato is null         
UNION select 'proof flh_id_messaggio is not null' description, count(1)
from eni_flussi_hub
where flh_id_messaggio is null

Answer 1

EXISTS / NOT EXISTS等效查询是通过将NULL条件放在WHERE子句中而不是OUTER JOIN子句中获得的。顺便说一下，这是我们从你的结果中观察到的：

No condition                             6403
is not null in where cond                6401
is null in where cond                       2

主表中的2行在连接表中没有相应的ID。

当你将条件放在OUTER JOIN子句中时，你要告诉Oracle将你的主表OUTER JOIN从连接表中行的子集。

由于c.flh_stato永远不为null，因此条件是多余的，我们得到与无条件查询相同的结果：

No condition                             6403
is NOT null in OUTER JOIN cond           6403

在join子句中使用条件c.flh_stato IS NULL，我们将主表连接到空结果集，因此我们为主表的每一行得到一行（我们推断主表有6247行，这种情况）：

is null in OUTER JOIN cond               6247

Oracle左外连接在JOIN和WHERE条件中为null（示例）

1 个答案: