使用子 SELECT 查询中的 COUNT 值来确定 FETCH ROW 限制

发布于 2024-10-19 03:32:50 字数 418 浏览 12 评论 0原文

我很好奇是否有任何方法可以重写以下无效的 DB2 查询以获得我需要的内容：

WITH pull AS (
    SELECT *
    FROM email_list
), 
pull_count AS (
    SELECT count(email) as email_count
    FROM email_list
)

SELECT *
FROM pull
FETCH FIRST integer(email_count / 2) ROWS ONLY

我收到“Token INTEGER was not valid”。有效令牌：ROW ROWS' 错误。

我的目标是使用第二个WITH 语句pull_count 为第一个WITH 语句pull 设置FETCH 限制。我只想选择一半可用的行。

有什么建议吗？

原文

I'm curious to see if there's any way to rewrite the following invalid DB2 Query to get what I need:

WITH pull AS (
    SELECT *
    FROM email_list
), 
pull_count AS (
    SELECT count(email) as email_count
    FROM email_list
)

SELECT *
FROM pull
FETCH FIRST integer(email_count / 2) ROWS ONLY

I'm getting the 'Token INTEGER was not valid. Valid tokens: ROW ROWS' error.

My goal is to use the 2nd WITH statement, pull_count, to set the FETCH limit for the 1st WITH statement, pull. I'm only looking to SELECT for half of the rows available.

Any suggestions?

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我一向站在原地 2024-10-26 03:32:50

正如您所发现的，FETCH FIRST 子句不能使用表达式。它仅接受非负整数。

没有一种很好的方法可以做到这一点，但可以通过使用 OLAP 函数来实现。这些可能不存在于您的 DB2 版本上（如果您使用的是 DB2 for Linux/UNIX/Windows，那么它们可能存在）。

假设您的表有电子邮件和姓名列：

with pull as (
   select 
      email, 
      name, 
      rownumber() over(order by email) as row, 
      count(email) over () as cnt
   from 
      email_list
)
select 
   email,
   name 
from 
   pull
where
   row <= cnt/2;

这可能表现不佳；事实上，简单地执行 2 个查询可能会更有效 - 一个执行计数，另一个获取您关心的特定行数。

The FETCH FIRST clause can't use an expression, as you've found. It accepts a non-negative integer only.

There is not a pretty way to do this, but it's possible by using OLAP functions. These may not be present on your version of DB2 (if you're on DB2 for Linux/UNIX/Windows then they are probably there).

Assuming your table has columns email and name:

with pull as (
   select 
      email, 
      name, 
      rownumber() over(order by email) as row, 
      count(email) over () as cnt
   from 
      email_list
)
select 
   email,
   name 
from 
   pull
where
   row <= cnt/2;

This may not perform very well; and in fact it may be more efficient to simply execute 2 queries -- 1 doing the count, and the other fetching the specific number of rows you care about.

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