SQL 显示GROUP BY中的最新记录？

Question

我有一个看起来像这样的表：

id    |    SubjectCode    |    Grade    |    DateApproved    |    StudentId

1            SUB123            1.25            1/4/2012            2012-12345

2            SUB123            2.00            1/5/2012            2012-12345

3            SUB123            3.00            1/5/2012            2012-98765   

我正在尝试 GROUP BY SubjectCode 但我希望它显示最新的 DateApproved 所以它看起来像：

  id    |    SubjectCode    |    Grade    |    DateApproved    |    StudentId

2            SUB123            2.00            1/5/2012            2012-12345

3            SUB123            3.00            1/5/2012            2012-98765  

我对如何去做有点迷茫？

编辑：

好的，伙计们，我现在在我的真实电脑上，对于构建不当的问题感到抱歉。

这是我实际上想做的事情：

SELECT GD.GradebookDetailId, G.SubjectCode, G.Description, G.UnitsAcademic, G.UnitsNonAcademic, 
GD.Grade, GD.Remarks, G.FacultyName, STR_TO_DATE(G.DateApproved, '%m/%d/%Y %h:%i:%s') AS 'DateAproved'
FROM gradebookdetail GD INNER JOIN gradebook G ON GD.GradebookId=G.GradebookId 
WHERE G.DateApproved IS NOT NULL AND G.GradebookType='final' AND StudentIdNumber='2012-12345'

GROUP BY <?????>
ORDER BY G.SubjectCode ASC

基本上，我只想显示“SubjectCode”的最新“DateApprove”，所以我不会获得多个条目。

Answer 1

从这个开始：

select StudentId, max(DateApproved) 
from tbl
group by StudentId

然后将其集成到主查询：

select * 
from tbl
where (StudentId, DateApproved) in

(
  select StudentId, max(DateApproved) 
  from tbl
  group by StudentId
)

你也可以使用这个：

select * 
from tbl
join (select StudentId, max(DateApproved) as DateApproved 
      from tbl 
      group by StudentId)
using (StudentId, DateApproved)

但我更喜欢元组测试，它更整洁

现场测试： http ://www.sqlfiddle.com/#!2/771b8/5

Answer 2

SELECT t2.*
FROM temp t2 
INNER JOIN(
    SELECT MAX(DateApproved) as MaxDate, StudentId
    FROM temp
    GROUP BY StudentId
    ) t1 ON t1.MaxDate = t2.DateApproved and t1.StudentId = t2.StudentId

Answer 3

对于在撰写本文时（2020 年）仍不支持元组的 SQL Server 中尝试此操作的用户，您可以使用以下内容（在此处改进另一个答案）

改变这个：

-- MYSQL
select * 
from tbl
where (StudentId, DateApproved) in
(
  select StudentId, max(DateApproved) 
  from tbl
  group by StudentId
)

进入这个：

-- Microsoft SQL Server
select * 
from tbl
where concat(StudentId, DateApproved) in
(
  select concat(StudentId, max(DateApproved)) 
  from tbl
  group by StudentId
)

Answer 4

SELECT *
FROM TheTable a
WHERE NOT EXISTS(SELECT *
                 FROM TheTable b
                 WHERE b.StudentCode = a.StudentCode AND b.DateApproved > a.DateApproved)

Answer 5

这对我在 postgres 中更有效。

select * from tbl t1 where t1.DateApproved = (select max(t2.DateApproved) from tbl t2 where t2.StudentId = t1.StudentId)

Answer 6

此代码适用于 SQL Server 2016。

select StudentId, max(DateApproved) 来自 tbl group by StudentId

此代码将显示每个学生 ID 批准的最大和最小日期。

select StudentId，最大值（批准日期），最小值（批准日期）
来自 tbl 按 StudentId 分组