sql - oracle:解码和子查询选择结果

Question

我有一个 oracle 查询，其中一部分是使用 DECODE 计算一些值。例如:

SELECT ..., 
      (SELECT DECODE((SELECT 23 FROM DUAL), 
                      0, null,
                     (SELECT 23 FROM DUAL))  
         FROM DUAL) 
  FROM ...

这里的值“23”是在运行时计算的，它是相当复杂的连接 - 多个表，使用 PARTITION BY等等。所以如果值不是“0”，我想避免执行相同的子查询。有没有办法写这样的东西

SELECT ..., 
       (SELECT DECODE ((SELECT 23 FROM DUAL) as test, 
                        0, null,
                       test)  
         FROM DUAL) 
  FROM ...

Answer 1

这对你有用吗？
我刚刚将“23”移动到带有描述性别名的内联表中。

select ..., 
  (
   select 
     decode ( 
            computed_value.val, 
            0, null,
            computed_value.val
            )  
   from
     (select 23 as val from dual) computed_value
  )
from
  ...

CASE 语句也可能会增加清晰度，例如:

select
  ...
 ,case when computed_value.val = 0
       then null
       else computed_value.val
       end as my_field
from
  (select 23 as val from dual) computed_value
  ...