Sql查询加入四个表

Question

SELECt 
    qst_id,qst_title,ans_date,ans_text 
FROM
    (
        SELECT 
            a.Question_Id as qst_id,a.Question_Title as qst_title,a.Question_Text as qst_text,DATE_FORMAT(a.LastActivity_Date,'%d %b %Y %T') as qst_date,b.UserForum_Image as qst_prof,b.ScreenName as qst_scname

        FROM 
            tblforumquestion a, tblregistration2_2 b 
        WHERE  a.RegistrationId=b.RegistrationId and a.LastActivity_Date between '2014-0-01 00:00:00' and '2015-05-01 00:00:00'
        ORDER BY a.LastActivity_Date desc limit 5

        outer join

        SELECT 
            DATE_FORMAT(c.Answer_Date,'%d %b %Y %T')  as ans_date,c.Answer_Text as ans_text,d.UserForum_Image as ans_prof,d.ScreenName as ans_scname
        FROM 
            tblforumanswer c ,tblregistration2_2 d
        where c.Answer_Id in 
            ( 
                SELECT  MAX(Answer_Id)
                FROM tblforumanswer
                GROUP BY Question_Id 
            )  
        and c.RegistrationId=d.RegistrationId 
        order by c.Answer_Date desc limit 5
    )

我试图从我的帖子中得到最新的5个问题和答案。如果没有答案的任何问题都存在，它也应该在一行中显示为具有空答案细节的问题详细信息。但是上面的代码正在收到错误。任何帮助都是明显的。我的数据库是mysql。

tblquest tblans结果

Answer 1

我想我们终于提取了足够的细节来得出答案：

select q.qstid, q.qsttext, a.anstext
  from tblquest q
    left join tblans a
      on q.qstid = a.qstid
    left join tblans a2
      on a.qstid = a2.qstid and a.ansdate < a2.ansdate
  where a2.ansdate is null
  order by q.qdate desc limit 5;

在这里演示

我们left join了问题的答案，以确保我们保留所有问题，包括那些没有答案的问题。

然后我们再次left join答案，但这次是在一个范围条件下，以便选择最近的问题答案。 如果没有a2的日期大于a ，则a必须是最近的答案 - 这将由where a2.ansdate is null子句的where a2.ansdate is null过滤。

如果您愿意，也可以使用子查询来完成。

最后，我们只是订购并限制我们的结果，以便获得最近的5个问题。

Answer 2

外连接语法有问题。 检查评论和样本数据。

SELECT
    qst_id,qst_title,ans_date,ans_text 
FROM
    (
        SELECT 
            a.Question_Id as qst_id,a.Question_Title as qst_title,a.Question_Text as qst_text,DATE_FORMAT(a.LastActivity_Date,'%d %b %Y %T') as qst_date,b.UserForum_Image as qst_prof,b.ScreenName as qst_scname

        FROM 
            tblforumquestion a, tblregistration2_2 b 
        WHERE  a.RegistrationId=b.RegistrationId and a.LastActivity_Date between '2014-0-01 00:00:00' and '2015-05-01 00:00:00'
        ORDER BY a.LastActivity_Date desc limit 5

        outer join  --Error comes here

        SELECT 
            DATE_FORMAT(c.Answer_Date,'%d %b %Y %T')  as ans_date,c.Answer_Text as ans_text,d.UserForum_Image as ans_prof,d.ScreenName as ans_scname
        FROM 
            tblforumanswer c ,tblregistration2_2 d
        where c.Answer_Id in 
            ( 
                SELECT  MAX(Answer_Id)
                FROM tblforumanswer
                GROUP BY Question_Id 
            )  
        and c.RegistrationId=d.RegistrationId 
        order by c.Answer_Date desc limit 5
    )

--This is example of outer join
SELECT 
    A.*, B.*
FROM 
    TableA a outer join TableB b on a.RegistrationId = b.RegistrationId

请参阅链接了解更多详情：

完全外部加入MySQL

https://dev.mysql.com/doc/refman/5.0/en/outer-join-simplification.html

http://www.w3schools.com/sql/sql_join_full.asp