Question

首先，我正在处理4个表。

我有一个类表，它是与部分表的1＆gt; N关系，该表与具有1＆gt; N关系课程表。

所以要把它放在一起：

类
- 的节
  - 的吸取

最后一个表是 activityLog ，当学生访问课程时，使用以下内容记录：

ActivityLog Row - ＆gt; actorID（用户ID），classID，sectionID，lessonID

我想提取学生访问过的最后5个独特课程。我尝试使用DISTINCT和GROUP BY都没有成功。

每次都会返回相同的记录，而不是他们访问过的最新类。

使用GROUP BY

SELECT activityLog.actorID, activityLog.activityDate,
        strClasses.classID, strClasses.className,
        strSections.sectionID, strSections.sectionName,
        strLessons.lessonID, strLessons.lessonName

FROM activityLog            
LEFT JOIN strClasses ON strClasses.classID = activityLog.classID
LEFT JOIN strSections ON strSections.sectionID = activityLog.sectionID
LEFT JOIN strLessons ON strLessons.lessonID = activityLog.lessonID
WHERE activityLog.activityTypeID = 6 AND activityLog.actorID = 3
GROUP BY activityLog.lessonID
ORDER BY activityLog.activityDate DESC
LIMIT 5

使用DISTINCT

SELECT DISTINCT activityLog.actorID,
        strClasses.classID, strClasses.className,
        strSections.sectionID, strSections.sectionName,
        strLessons.lessonID, strLessons.lessonName
FROM activityLog            
LEFT JOIN strClasses ON strClasses.classID = activityLog.classID
LEFT JOIN strSections ON strSections.sectionID = activityLog.sectionID
LEFT JOIN strLessons ON strLessons.lessonID = activityLog.lessonID
WHERE activityLog.activityTypeID = 6 AND activityLog.actorID = 3
ORDER BY activityLog.activityDate DESC
LIMIT 5

我无法弄清楚为什么没有显示最新的记录。

Answer 1

根据您的更改，这对您有何帮助？

SELECT activityLog.actorID, activityLog.activityDate,
        strClasses.classID, strClasses.className,
        strSections.sectionID, strSections.sectionName,
        strLessons.lessonID, strLessons.lessonName
FROM activityLog            
LEFT JOIN strClasses ON strClasses.classID = activityLog.classID
LEFT JOIN strSections ON strSections.sectionID = activityLog.sectionID
LEFT JOIN strLessons ON strLessons.lessonID = activityLog.lessonID
WHERE activityLog.activityTypeID = 6 AND activityLog.actorID = 3
AND activityLog.activityDate = (SELECT MAX(activityDate) FROM activityLog AS lookup WHERE lessonID = activityLog.lessonID)
ORDER BY activityLog.activityDate DESC
LIMIT 5

根据您的描述，我不确定您使用LEFT JOIN的原因，但为了以防万一，我已将其保留。

Answer 2

请按以下方式尝试分组 GROUP BY activityLog.classID，activityLog.sectionID，activityLog.lessonID

我认为它会起作用，或者只是发送给我创建这些脚本我将创建该查询

Answer 3

嗯，我希望ActivityLog中有一个日期时间......所以试试这个：

 Select s.Name, c.ClassName
 From Students s
     left Join On Classes c
          On c.ClassId In 
               (Select Distinct ClassId From Classes
                Where (Select Count(Distinct ClassId) From Classes ic
                         Join ActivityLog l On l.UserId = s.UserId
                            And l.ClassId = c.ClassId                         
                       Where classId = c.ClassId
                          And activityDateTime > l.activityDateTime) 
                    < 5)

MySQL SELECT DISTINCT ORDER BY问题

3 个答案: