SQL计算列

Question

我第一次使用SAP HANA。以下是我计算视图的一些示例数据。我想要的结果是计算后的最后一位老师。

我有2个学生。有一个学生最后一次见过老师的日期。对于每个学生，我想用sql语句计算上次见过的老师。有什么想法吗？

Answer 1

在目前不支持窗口功能的MySQL中，你可以试试这个：

SELECT
    t1.*,
    t3.TeacherID AS last_teacher
FROM yourTable t1
INNER JOIN
(
    SELECT studentID, MAX(LastSeen) AS max_last_seen
    FROM yourTable
    GROUP BY studentID
) t2
    ON t1.studentID = t2.studentID
INNER JOIN yourTable t3
    ON t2.studentID     = t3.studentID AND
       t2.max_last_seen = t3.LastSeen;

使用窗口函数，我们可以取消其中一个连接，并稍微简化以下内容：

SELECT
    t1.StudentID,
    t1.TeacherID,
    t1.LastSeen,
    t2.TeacherID AS last_teacher
FROM
(
    SELECT StudentID, TeacherID, LastSeen,
        MAX(LastSeen) OVER (PARTITION BY StudentID) AS max_last_seen
    FROM yourTable
) t1
INNER JOIN yourTable t2
    ON t1.StudentID     = t2.StudentID AND
       t1.max_last_seen = t2.LastSeen;

Answer 2

尝试以下查询。

SELECT *
FROM table
WHERE last_seen IN (
  SELECT Max(last_seen)
  FROM table
  GROUP BY student_id
)

Answer 3

您可以使用Row_Number()函数在oracle db中按列获取LastRecord，如下所示：

Select * from
(
  select 
    ROW_NUMBER() OVER (PARTITION BY studentId ORDER BY LastSeen desc) as rn,
    studentId,
    TeacherId
  from yourTable
)
where rn = 1;

此功能将每个studentId分组，并为每个studentIdGroup按LastSeen列排序。然后，您可以使用where语句where rn = 1

为每个组选择第一行

<强>更新 如果你想要选择表格中的所有行，那么你可以将这个选择声明加入你的表格，如下所示：

select * from yourTable t
join 
(
  Select * from
  (
    select 
      ROW_NUMBER() OVER (PARTITION BY studentId ORDER BY LastSeen desc) as rn,
      studentId,
      TeacherId
    from yourTable
  )
  where rn = 1
) s
on t.studentId = s.studentId;