Oracle SELECT TOP 10记录 [英] Oracle SELECT TOP 10 records

问题描述

我在Oracle中的SQL语句有很大的问题.我想选择STORAGE_DB排序的前10条记录，这些记录不在其他select语句的列表中.

I have an big problem with an SQL Statement in Oracle. I want to select the TOP 10 Records ordered by STORAGE_DB which aren't in a list from an other select statement.

此记录适用于所有记录:

This one works fine for all records:

SELECT DISTINCT 
  APP_ID, 
  NAME, 
  STORAGE_GB, 
  HISTORY_CREATED, 
  TO_CHAR(HISTORY_DATE, 'DD.MM.YYYY') AS HISTORY_DATE  
  FROM HISTORY WHERE 
      STORAGE_GB IS NOT NULL AND 
        APP_ID NOT IN (SELECT APP_ID
                       FROM HISTORY
                        WHERE TO_CHAR(HISTORY_DATE, 'DD.MM.YYYY') = '06.02.2009') 

但是当我添加

AND ROWNUM <= 10
ORDER BY STORAGE_GB DESC

我正在获取某种随机"记录.我认为是因为限价是在下订单之前就完成的.

I'm getting some kind of "random" Records. I think because the limit takes in place before the order.

有人有一个好的解决方案吗?另一个问题:该查询的速度非常慢(超过10k条记录)

Does someone has an good solution? The other problem: This query is realy slow (10k+ records)

推荐答案

您需要将当前查询放入子查询中，如下所示:

You'll need to put your current query in subquery as below :

SELECT * FROM (
  SELECT DISTINCT 
  APP_ID, 
  NAME, 
  STORAGE_GB, 
  HISTORY_CREATED, 
  TO_CHAR(HISTORY_DATE, 'DD.MM.YYYY') AS HISTORY_DATE  
  FROM HISTORY WHERE 
    STORAGE_GB IS NOT NULL AND 
      APP_ID NOT IN (SELECT APP_ID FROM HISTORY WHERE TO_CHAR(HISTORY_DATE, 'DD.MM.YYYY') ='06.02.2009')
  ORDER BY STORAGE_GB DESC )
WHERE ROWNUM <= 10

Oracle返回结果后，将 rownum 应用于结果.
您需要在返回结果后对其进行过滤，因此需要一个子查询.您还可以使用 RANK()函数来获取Top-N结果.

为了提高性能，请尝试使用NOT EXISTS代替NOT IN.有关更多信息，请参见此.

Oracle applies rownum to the result after it has been returned.
You need to filter the result after it has been returned, so a subquery is required. You can also use RANK() function to get Top-N results.

For performance try using NOT EXISTS in place of NOT IN. See this for more.

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