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

每个表格行每天都会使用当前时间戳更新。

在识别方面需要帮助，Mostrecentdate列应该在比较这两个列的两个不同的表和两个不同的列之后得到日期。如果在activitydata和assessmentdata中列都为null，则在具有mostrecentdate的预期输出为null，如图所示。

左外连接应该是studentdata表，我应该在哪里写这个？

预期输出应为，

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

Answer 2

试试这个配偶：

SELECT SD.StudentID
    , SD.StudentName
    , CASE
        WHEN MAX(IFNULL(SS.attemptedondate, '')) >= MAX(IFNULL(AC.date, '')) THEN MAX(SS.attemptedondate)
        WHEN MAX(IFNULL(SS.attemptedondate, '')) < MAX(IFNULL(AC.date, '')) THEN MAX(AC.date)
        ELSE NULL
      END AS MOSTRECENTDATE
    FROM Studentdata SD
    LEFT JOIN Assessmentdata SS ON SS.StudentID = SD.StudentID
    LEFT JOIN Activitydata AC ON AC.StudentID = SD.StudentID
    GROUP BY SD.StudentID;

这是SQLFiddle Demo。

左外连接与案例条件

2 个答案: