mysql - 带案例条件的左外连接

Question

我的数据库有三个表，

Studentdata with columns studentid , studentname
Assessmentdata with columns studentid, attemptedondate
Activitydata with columns studentid, date

并且每个表行将每天使用当前时间戳进行更新。

需要帮助识别，最近的日期列应该在比较两个不同的表和两个不同的列中的 attemptedondate 和 date 这两个列之后获取日期。如果该列在 activitydata 和 assessmentdata 中都为空，则在如图所示，最近日期的预期输出为空。

left outer join应该是studentdata表，应该写在哪里？

预期的输出应该是，

Studentid   studentname      mostrecentdate
 1           abc            2013-06-01 12:05
 2           def            2013-05-02 02:03
 3           kjr                  null

My current query is:
select S.StudentId,
S.StudentAccountName,
CASE WHEN Max(D.attemptedondate) >= Max(A.date) THEN Max(D.attemptedondate) 
     ELSE Max(A.date) END
 as MOSTRECENTDATE
from activitydata A
join Studentdata S on A.StudentId=s.StudentID
join Assessmentdata D on S. StudentID =D. StudentID
 group by S.StudentId,
S.StudentAccountName

Answer 1

尝试

SELECT s.studentid, s.studentname, 
       NULLIF(GREATEST(COALESCE(a.max_date, 0), 
                       COALESCE(b.max_date, 0)), 0) mostrecentdate
  FROM Studentdata s LEFT JOIN
(
  SELECT studentid, MAX(attemptedondate) max_date
    FROM Assessmentdata
   GROUP BY studentid
) a ON s.studentid = a.studentid LEFT JOIN
(
  SELECT studentid, MAX(date) max_date
    FROM Activitydata
   GROUP BY studentid
) b ON s.studentid = b.studentid

或

SELECT s.studentid, s.studentname, mostrecentdate
  FROM Studentdata s LEFT JOIN
(
  SELECT studentid, MAX(max_date) mostrecentdate
    FROM
  (
    SELECT studentid, MAX(attemptedondate) max_date
      FROM Assessmentdata
     GROUP BY studentid
     UNION ALL
    SELECT studentid, MAX(date) max_date
      FROM Activitydata
     GROUP BY studentid
  ) a 
   GROUP BY studentid
) b ON s.studentid = b.studentid

示例输出:

| STUDENTID | STUDENTNAME |      MOSTRECENTDATE |-------------------------------------------------|         1 |         abc | 2013-06-01 12:05:00 ||         2 |         def | 2013-05-02 02:03:00 ||         3 |         kjr |              (null) |

这是 SQLFiddle 演示