SQL - 我需要限制从第二个表返回的行？

Question

我正在使用SQL Server 2008。

我有一个问题表，我有一个答案表。问题表包含QuestionID，Question和Position（order）。 Answers表包含Answers和AnswerID。每个问题都可以多次回答。

我们的顾问应该只看到最近的答案，这就是我们按答案ID DESC排序的原因。

我们的管理员需要能够查看每个答案的历史记录。这就是为什么我们不删除答案。

我的查询现在看起来像这样：

 SELECT   Q.*, A.*
 FROM     Questions Q 
          LEFT JOIN Answers A 
               ON Q.QuestionID = A.QuestionID
 WHERE      FranchisorID = 10   
 ORDER BY   Q.Position, A.AnswerID DESC

上面查询的结果集如下所示：

QuestionID - Question - AnswerID - Answer - 

 1 what is your fave color? 3 Blue
 1 what is your fave color? 2 Green
 1 what is your fave color? 1 Red
 4 What year is this? 5 2011
 4 What year is this? 4 2010

我需要将结果集看起来像这样：

 1 what is your fave color? 3 Blue
 4 What year is this? 5 2011

你能为我调整一下我的小查询吗？

Answer 1

一种选择是

• 添加ROW_NUMBER
• PARTITION QuestionID
{li} ORDER BY DESC AnswerID只返回最大的AnswerID
• 返回rn = 1
的行

SQL语句

SELECT  *
FROM    (
            SELECT  Q.QuestionID
                    , Q.Question
                    , A.AnwerID
                    , A.Answer
                    , rn = ROW_NUMBER() OVER (PARTITION BY QuestionID ORDER BY AnswerID DESC)
            FROM    Questions Q 
                    LEFT JOIN Answers A ON Q.QuestionID = A.QuestionID
             WHERE  FranchisorID = 10
        ) r
WHERE   r.rn = 1                            

Answer 2

• 使用CTE获取最新的答案ID

• 使用INNER JOIN到CTE过滤结果。

  WITH LatestAnswer AS
  (
      SELECT  QuestionID,
              MAX(AnswerID) AS LatestAnswerID
      FROM    Questions
      INNER JOIN Answers ON Answers.QuestionID = Questions.QuestionID
      GROUP BY QuestionID
  )
  
  SELECT      Q.*, A.*
  FROM        Questions Q 
  LEFT JOIN   Answers A ON Q.QuestionID = A.QuestionID
  INNER JOIN  LatestAnswer ON LatestAnswer.LatestAnswerID = Answers.AnswerID
  WHERE       FranchisorID = 10   
  ORDER BY    Q.Position, A.AnswerID DESC
  

Answer 3

 SELECT   Q.*, A.*
 FROM     Questions Q 
   LEFT JOIN Answers A 
   ON A.AnswerId in 
     (select top 1 a2.AnswerId from Answers a2 
      where a2.QuestionId=Q.QuestionId order by a2.AnswerId DESC)
 WHERE      FranchisorID = 10   
 ORDER BY   Q.Position