左外连接与情况

Question

My database has three tables,

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

And each table row will be updated every day with the current timestamp.

Need help in identifying, Mostrecentdate column should get the date after comparing both these columns attemptedondate and date from two different tables and two different columns.if the column is null in both the activitydata and assessmentdata then print the student id with in the expected output with mostrecentdate is null as shown.

Left outer join should be with studentdata table , where should I write this ?

Expected output should be,

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

Try

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

or

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

Sample output:

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

Here is SQLFiddle demo

Answer 2

Try this one mate:

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;

Here is the SQLFiddle Demo .