Oracle左外部联接在联接vs WHERE条件中为null（示例）

我无法理解Oracle返回这种奇怪结果的原因。 我认为代码真的很清楚

我预料 无条件=（在外部联接条件中不为null）+（在外部联接条件中为null）

因为我将外部联接右表列中的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  

---

存在

/

不存在

等效查询是通过将

NULL

条件放置在

WHERE

子句中而不是

外部联接

子句中获得的。顺便说一下，这是我们从您的结果中观察到的：

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

主表中的2行在联接表中没有相应的ID

---

当您在

外部联接

子句中放置条件时，您告诉Oracle将

外部联接

您的主表从联接表中转到一个行子集

由于

c.flh_stato

从不为空，因此该条件是冗余的，我们得到的结果与无条件查询相同：

No condition                             6403
is NOT null in OUTER JOIN cond           6403

如果join子句中的条件

c.flh_stato为NULL

，我们将主表连接到一个空结果集，因此主表的每行得到一行（我们推断主表有6247行具有此条件）：

---

真的很高兴知道。即使我觉得甲骨文很奇怪。。。为什么允许在联接条件中预筛选表。。真奇怪。。我认为在连接条件中只使用连接条件，可能使用EXIST/NOT EXIST等效项。。不是预过滤器…@Gik25我不确定这是Oracle特有的。这似乎是SQLSynthax的一个直接应用程序。请参见中的这些行为类似的示例。通常情况下，使用

NOT IN

或

NOT EXISTS

的反连接更容易理解，并且在Oracle中具有相同的优化器选项（不过要注意空值）。

is null in OUTER JOIN cond               6247