SQL查询与SQL子查询

Question

大家好，我编写了以下查询，以合并3个表的结果（ tblQuestions,tblReplies,tblTechnology并显示所需结果，如下所示）

SELECT tech.DatePosted,
       tech.QuestionID,
       tech.TechnologyName,
       tblr3.TechID,
       tblr3.QuestionTitle,
       tblr3.UserName,
       tblr3.ReplyCount,
       tblr3.viewCount,
       tblr3.date,
       tblr3.RepliedName
FROM   (SELECT tblr1.DatePosted,
               tblr1.QuestionID,
               tblr2.TechID,
               tblr2.QuestionTitle,
               tblr2.UserName,
               tblr2.ReplyCount,
               tblr2.viewCount,
               tblr1.date,
               tblr1.RepliedName
        FROM   (SELECT tblq.DatePosted,
                       tblq.TechID,
                       tblq.QuestionID,
                       tblq.RepliedName,
                       tblq.QuestionTitle,
                       tblq.UserName,
                       tblq.ReplyCount,
                       tblq.viewCount,
                       tblq.date
                FROM   (SELECT q.DatePosted,
                               q.TechID,
                               q.QuestionID,
                               q.QuestionTitle,
                               q.UserName,
                               q.ReplyCount,
                               q.viewCount,
                               r.date,
                               r.UserName AS RepliedName
                        FROM   tblQuestions AS q, tblReplies AS r
                        WHERE  r.QuestionID = q.QuestionID
                               AND r.TechID = q.TechID
                               AND q.TechID = 1) AS tblq,
                                                 (SELECT   r.QuestionID,
                                                           max(r.date) AS PostedDate
                                                  FROM     tblReplies AS r
                                                  GROUP BY QuestionID) AS tblr
                WHERE  tblq.QuestionID = tblr.QuestionID
                       AND tblq.date = tblr.PostedDate) AS tblr1
               RIGHT OUTER JOIN
               (SELECT q.QuestionID,
                       q.TechID,
                       q.UserName,
                       q.viewCount,
                       q.ReplyCount,
                       q.QuestionTitle
                FROM   tblQuestions AS q
                WHERE  q.TechID = 1) AS tblr2
               ON tblr2.QuestionID = tblr1.QuestionID) AS tblr3
       LEFT OUTER JOIN
       (SELECT q.QuestionID,
               q.DatePosted,
               t.TechID,
               t.TechName AS TechnologyName
        FROM   tblTechnology AS t, tblQuestions AS q
        WHERE  q.TechID = t.TechID) AS tech
        ON tblr3.TechID = tech.TechID
           AND tblr3.QuestionID = tech.QuestionID
           AND tech.TechID = 1

这给了我以下预期的结果，但是在某些要显示数据的字段上为NULL

代替NULL我需要提取tblQuestions存在的一些数据，也有可能减少查询。

从OP评论：

我正在尝试uestiontitle,username,questionid from tblquestions提取q uestiontitle,username,questionid from tblquestions以及replycount and viewcount uestiontitle,username,questionid from tblquestions以及基于replycount and viewcount QuestionID和Technology ID针对该问题发布的最新tblreplys

Answer 1

您的查询基本上是这样的：

SELECT
    blah
FROM
    (
        (SELECT
            blah
        FROM
            Questions
            INNER JOIN Replies ON QuestionID AND TechID) tblr1
        RIGHT JOIN Questions tblr2 ON QuestionID
    ) tblr3
    LEFT JOIN Technologies ON tblr3.tblr1.QuestionID = Technologies.QuestionID

我知道这不是有效的查询，但请多多包涵。 tblr1在问题和答案之间包含一个INNER JOIN 。 但是，由于没有salary问题的答案，因此这些记录对于salary问题为NULL 。 然后，对问题的“ RIGHT JOIN将引入salary问题，并且该问题存在于tblr3结果集中，因为即使该问题的tblr1为空，它也是“ RIGHT JOIN 。

该LEFT JOIN然后技术是基于QuestionID内tblr3这是真正的QuestionID从tblr1 。 通过查看tblr3子查询的列列表，如果将tblr1.QuestionID更改为tblr2.QuestionID这将确保将未回答的问题加入到Technology表中

编辑：我认为以下查询将为您提供结果，并且更加简单：

WITH LatestReplies AS
(
  SELECT
    ReplyID,
    QuestionID,
    ReplyDate,
    UserName,
    Reply
  FROM
    (SELECT
      ReplyID,
      QuestionID,
      ReplyDate,
      UserName,
      Reply,
      ROW_NUMBER() OVER (PARTITION BY QuestionID ORDER BY ReplyDate DESC) ReplyOrder
    FROM
      tblReplies) A
  WHERE
    ReplyOrder = 1
)
SELECT
  tblQuestions.DatePosted,
  tblQuestions.QuestionID,
  tblTechnology.TechnologyName,
  tblQuestions.TechID,
  tblQuestions.QuestionTitle,
  tblQuestions.UserName,
  tblQuestions.ReplyCount,
  tblQuestions.ViewCount,
  LatestReplies.ReplyDate,
  LatestReplies.UserName RepliedName
FROM
  tblQuestions
  LEFT JOIN tblTechnology ON tblQuestions.TechID = tblTechnology.TechID
  LEFT JOIN LatestReplies ON tblQuestions.QuestionID = LatestReplies.QuestionID

Answer 2

毕竟解决了以下问题

select 
    FI.QuestionID,
    FI.QuestionTitle,
    FI.UserName,
    FI.DatePosted,
    FI.[date],
    FI.RepliedName,
    FI.viewCount,
    FI.ReplyCount,
    FI.ReplyMsg,
    TT.TechID,
    TT.TechName 
from 
    tblTechnology TT,
    (select distinct
        TQ.TechID,
        TQ.QuestionID,
        TQ.QuestionTitle,
        TQ.UserName,
        TQ.DatePosted,
        TR.[date],
        TR.UserName as RepliedName,
        TQ.viewCount,
        TQ.ReplyCount,
        TR.ReplyMsg 
    from  
        tblQuestions TQ 
        LEFT OUTER JOIN tblReplies TR ON 
                TR.TechID=TQ.TechID 
            and TR.QuestionID = TQ.QUESTIONID 
            and TR.[date] in (select 
                                MAX(TR.[date]) 
                            from 
                                tblReplies TR 
                            group by 
                                TR.QuestionID)
    ) FI 
where 
    FI.TechID=TT.TechID 
and TT.TechID=@TechID