Question

我必须通过某些列来整理我的数据，以便首先显示某些特定值。所以对于像这样的查询...

  SELECT rtrim(taskid) into v_taskid FROM tasks
  where
    /* some where clausers */
  and rownum = 1

...我的基础是case when，但困扰我的是我现在拥有的三个嵌套选择：

  SELECT rtrim(taskid) into v_taskid FROM tasks where taskid in (
    select taskid from (
        select taskid,
        case when taskuser like '%myuser%' 
       then 0
           else 100
        end as ordervalue
        FROM tasks
        where       
          /* some where clausers */
        order by ordervalue
        )
    )    
    and rownum = 1

表现方面我认为它不应该是一个问题，但它看起来像一个意大利面......有没有办法将case-when放入where条款？

Answer 1

您要查询tasks表两次。这不是必需的，会使您的查询运行缓慢。您的查询可以组合成一个简单的查询，如下所示：

SELECT rtrim(taskid) into v_taskid
FROM (
  SELECT taskid 
  FROM tasks 
  WHERE /* some where clauses */
  ORDER BY case when taskuser like '%myuser%' then 0 else 100 end
)
WHERE rownum = 1;

然后，只需检索第一行或在外部查询中添加rownum部分。

Answer 2

用户row_number() over (...)

select 
  taskid
from (
  select 
    taskid,
    row_number() over (
      order by 
        case when taskuser like '%myuser%' then  0 
                                           else 100 
      end
    ) r
  from
    tasks
  WHERE
    /* some where clausers */
)
where 
  r = 1;

Answer 3

如果taskid是tasks的主键，则不需要自联接（外部SELECT）：

SELECT rtrim(taskid)
  FROM (SELECT taskid
           FROM tasks
           /* where clause */
          ORDER BY CASE
                      WHEN taskuser LIKE '%myuser%' THEN
                       0
                      ELSE
                       100
                   END)
 WHERE ROWNUM = 1

Oracle SQL案例何时进行订购

3 个答案: