sql - SQL Server：将子选择查询转换为加入

Question

我有 2 张两张桌子questionpool，question其中question一张是多张桌子question pool。我使用子选择查询创建了一个查询，该查询返回正确的随机结果，但我需要从表中返回多个列question。

查询的目的是从“问题”表中为“问题池”表中的每个“测验ID”返回一个随机测试。

SELECT QuestionPool.QuestionPoolID,
(
SELECT TOP (1) Question.QuestionPoolID
FROM Question
WHERE Question.GroupID = QuestionPool.QuestionPoolID
ORDER BY NEWID()
)
FROM QuestionPool
WHERE QuestionPool.QuizID = '5'

Answer 1

OUTER APPLY 适用于此：

Select *
FROM QuestionPool
OUTER APPLY
(
    SELECT TOP 1 * 
    FROM Question
    WHERE Question.GroupID = QuestionPool.QuestionPoolID
    ORDER BY NEWID()
) x
WHERE QuestionPool.QuizID = '5'

OUTER APPLY 的另一个示例使用http://www.ienablemuch.com/2012/04/outer-apply-walkthrough.html

---

现场测试：http ://www.sqlfiddle.com/#!3/d8afc/1

create table m(i int, o varchar(10));
insert into m values
(1,'alpha'),(2,'beta'),(3,'delta');

create table x(i int, j varchar, k varchar(10));

insert into x values
(1,'a','hello'),
(1,'b','howdy'),
(2,'x','great'),
(2,'y','super'),
(3,'i','uber'),
(3,'j','neat'),
(3,'a','nice');


select m.*, '' as sep, r.*
from m
outer apply
(
  select top 1 *
  from x
  where i = m.i
  order by newid()
) r

Answer 2

不熟悉 SQL 服务器，但我希望这样做：

Select QuestionPool.QuestionPoolID, v.QuestionPoolID, v.xxx -- etc
FROM   QuestionPool
JOIN
       (
        SELECT   TOP (1) *
        FROM     Question
        WHERE    Question.GroupID = QuestionPool.QuestionPoolID
        ORDER BY NEWID()
       ) AS v ON v.QuestionPoolID = QuestionPool.QuestionPoolID
WHERE  QuestionPool.QuizID = '5'

Answer 3

您的查询似乎为每个 QuestionPool.QuestionPoolId 带回了一个任意 Question.QuestionPoolId 受 QuizId 过滤器的影响。

我认为以下查询可以做到这一点：

select qp.QuestionPoolId, max(q.QuestionPoolId) as any_QuestionPoolId
from Question q join
     qp.QuestionPoolId qp
     on q.GroupId = qp.QuestionPoolId
 WHERE QuestionPool.QuizID = '5' 
group by qp.QuestionPoolId

这会返回一个特定的问题。

以下查询将允许您获取更多字段：

select qp.QuestionPoolId, q.*
from (select q.*, row_number() over (partition by GroupId order by (select NULL)) as randrownum
      from Question q
     ) join
     (select qp.QuestionPoolId, max(QuetionPool qp
     on q.GroupId = qp.QuestionPoolId
 WHERE QuestionPool.QuizID = '5' and
       randrownum = 1

这使用 row_number() 来任意枚举行。“Select NULL”提供随机排序（或者，您可以使用“order by GroupId”。

Answer 4

公用表表达式 (CTE) 对于这类事情非常方便......

http://msdn.microsoft.com/en-us/library/ms175972(v=sql.90).aspx