Question

我有以下MySQL表（student_lesson_progress）：

Id
lessonId
studentId
lessonStatusTypeId
recordDateTime
...

我需要记录当前学年的学生人数，他们的最后状态是两个值（6或13）。

我是SQL新手。我想我想从每个studentId / lessonId组合的最后一条记录开始。然后我想将该列表缩小到符合我标准的列表。这是我认为我想要的查询，但它在子查询中的“Max”处触发语法错误。

SELECT M1.*  
FROM student_lesson_progress M1  
{  
SELECT MAX(id) as lastRecord, lessonId, studentId, lessonStatusTypeId  
FROM student_lesson_progress  
GROUP BY studentId, lessonId  
} M2  
WHERE recordDateTime > "2012-08-01" 
AND (lessonStatusTypeId = 13 or lessonStatusTypeId = 6)  
ORDER BY studentId, lessonId, lastRecord

我很难找到具有计算值的子查询的语法示例。建议？或者也许有更好的方法？

Answer 1

您似乎错过了M1和M2

之间的联接

SELECT COUNT(M1.StudentID) AS Students
FROM student_lesson_progress M1
    INNER JOIN
    (SELECT MAX(id) as lastRecord, lessonId, studentId, lessonStatusTypeId  
     FROM student_lesson_progress  
     GROUP BY studentId, lessonId) M2 ON M1.ID = M2.lastRecord -- need to join M1 and M2 together
WHERE recordDateTime > '2012-08-01'
AND (lessonStatusTypeId = 13 or lessonStatusTypeId = 6)
ORDER BY studentId, lessonId, lastRecord

Answer 2

如果ID总是在增加，您可以使用此查询：

SELECT COUNT(*)
FROM student_lesson_progress
WHERE
  id IN (SELECT max(id)
         FROM student_lesson_progress
         GROUP BY studentId, lessonId)
  AND recordDateTime > '2012-08-01'
  AND lessonStatusTypeId IN (13, 6)

在子查询中使用MAX等计算值

2 个答案: