Oracle左外连接：如何限制右表中的请求

发布于 2024-09-18 14:01:48 字数 377 浏览 7 评论 0原文

我有一个很大的声明：

SELECT
    a.user_id, a.user_name,
    s.name, s.value,
    d.default
FROM
    accounts a,
    settings s
    LEFT OUTER JOIN default d ON ( d.name = s.name )
WHERE
    s.user_id = a.user_id;

问题是 settings 包含大量条目，我需要选择 ID 最高的条目。我可以想象更改语句并用子选择替换连接（它从 settings 获取正确的条目），但我很好奇是否有更好的解决方案。任何意见都会受到赞赏。非常感谢！

原文

I have a large statement:

SELECT
    a.user_id, a.user_name,
    s.name, s.value,
    d.default
FROM
    accounts a,
    settings s
    LEFT OUTER JOIN default d ON ( d.name = s.name )
WHERE
    s.user_id = a.user_id;

The problem is that settings contains a large amount of entries and I need to pick the one with the highest ID. I can imagine do change the statement and replace the join with a subselect (which grabs the correct entry from settings), but I'm curious to see if there is a better solution. Any input is appreciated. Many thanks!

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不气馁 2024-09-25 14:01:48

您可以使用子查询从右表中获取所需的结果。

像这样的东西：

SELECT
    *
FROM
    accounts a,
    (
      SELECT
          user_id,
          *
      FROM
          settings
      WHERE
          RANK() OVER (ORDER BY id DESC, PARTITION BY user_id) = 1
    ) s

You can use a sub-query to get just the results you want out of the right table.

Something like:

SELECT
    *
FROM
    accounts a,
    (
      SELECT
          user_id,
          *
      FROM
          settings
      WHERE
          RANK() OVER (ORDER BY id DESC, PARTITION BY user_id) = 1
    ) s

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美人骨 2024-09-25 14:01:48

从表中获取最高的 ID 可以使用 a

select max(id) ...

或 with a

select id from settings where rownum=1 order by id desc

来完成（我更喜欢第一个解决方案）
或者就像约翰提议的那样，但无论如何你都需要一个子查询。
另一件事，您的示例中可能存在一些拼写错误，我不知道 d 来自哪里，也没有进行自动左连接的要点...

这是我会写的

SELECT
    user_id, user_name,
    name, value,
    default
FROM
    accounts join 
    (select user_id,name 
     from settings 
     where RANK() OVER (ORDER BY id DESC, PARTITION BY user_id) = 1) using(user_id)
    LEFT OUTER JOIN default using ( name )

...仍然是子查询

getting the highest ID from a table could be done with a

select max(id) ...

or with a

select id from settings where rownum=1 order by id desc

(i prefer the first solution)
or just like John proposed, but you'll need a subquery anyway.
One more thing, there might be some typo in your example, i don't see where d is coming from, neither the point of making an auto left join...

Here is what I would have written

SELECT
    user_id, user_name,
    name, value,
    default
FROM
    accounts join 
    (select user_id,name 
     from settings 
     where RANK() OVER (ORDER BY id DESC, PARTITION BY user_id) = 1) using(user_id)
    LEFT OUTER JOIN default using ( name )

... still subquery

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