与SQL子查询

Question

大家好我写了下面的查询结果从3个表（合并tblQuestions,tblReplies,tblTechnology和如下

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 

这给了我下面的结果如预期，但得到NULL显示所需要的结果的SQL查询在某些领域，其中数据为存在

代替的NULL我需要拉出一些数据，其存在于tblQuestions，也是否有可能减少查询。

从OP的评论：

我试图用replycount and viewcount沿拉出第q uestiontitle,username,questionid from tblquestions和张贴基于QuestionID和技术的ID问题，从tblreplys

Answer 1

最新的回信您的查询基本上是这个：

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 

我知道这不是一个有效的查询，但忍受着我。 tblr1包含问题和答案之间的INNER JOIN。但是因为salary问题没有答案，所以这些记录salary问题的NULL。问题的RIGHT JOIN然后引入salary问题，并存在于tblr3结果集中，因为它是RIGHT JOIN，即使tblr1对于此问题为空。

的LEFT JOIN再到技术是基于内tblr3的QuestionID这是真正的QuestionID从tblr1。通过观察在列的列表为tblr3子查询，如果你改变tblr1.QuestionID到tblr2.QuestionID这将确保悬而未决的问题加入到Technology表

编辑：我觉得下面的查询会给你的结果和简单得多：

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

解决了毕竟

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 

以下