SQL - 如何在特定列中選擇具有最大值的數據行？

Question

在准備創建視圖時測試 SQL 查詢的過程中，我第一次想到了這個公式

select a.SchID as [SchID],  b.CourseID as [CourseID], a.EmpNo as [EmpNo], 
   b.ActStartDate as [ActStartDate], a.ValidTo as [ValidTo]
from [SQL3].[dbo].[Training_Record] a inner join 

(select z.schid, z.CourseID as [CourseID],max(z.ActStartDate) as [ActStartDate]
from [SQL3].[dbo].[Training_Schedule] z group by z.SchID, z.CourseID)as b

    on a.SchID = b.SchID 

-- to test data content
where EmpNo = '141281' and CourseID = '22'

結果給了我 2 行：

| SchID | CourseID | EmpNo |       ActStartDate      |         ValidTo         |
--------------------------------------------------------------------------------
| 5000  |    22    | 14000 | 2018-06-11 00:00:00.000 | 2018-12-10 00:00:00.000 |
| 5022  |    22    | 14000 | 2018-08-08 00:00:00.000 | 2019-02-07 00:00:00.000 |

我想要第 2 行，其 ActStartDate 是最大的，是唯一出現的行。

| SchID | CourseID | EmpNo |       ActStartDate      |         ValidTo         |
--------------------------------------------------------------------------------
| 5022  |    22    | 14000 | 2018-08-08 00:00:00.000 | 2019-02-07 00:00:00.000 |

但是由於 SchID 具有不同的編號，它最終注冊為單獨的條目。 SchID 是這兩個表中唯一共享的列，那么我如何告訴 SQL 忽略 SchID 讀取並僅顯示第二行？

Answer 1

您可以嘗試以下 - 使用子查詢

select a.SchID as [SchID],  b.CourseID as [CourseID], a.EmpNo as [EmpNo], 
   b.ActStartDate as [ActStartDate], a.ValidTo as [ValidTo]
from [SQL3].[dbo].[Training_Record] a 
inner join

(select z.schid, z.CourseID as [CourseID],max(z.ActStartDate) as [ActStartDate]
from [SQL3].[dbo].[Training_Schedule] z group by z.CourseID)as b
on a.SchID = b.SchID 

where EmpNo = '141281' and CourseID = '22' and 
b.ActStartDate in (select max([ActStartDate]) from [SQL3].[dbo].[Training_Schedule] z1 where b.courseid=z1.courseid group by z1.courseid)

或者您可以嘗試使用row_number()

select * from 
(
select a.SchID as [SchID],  b.CourseID as [CourseID], a.EmpNo as [EmpNo], 
   b.ActStartDate as [ActStartDate], a.ValidTo as [ValidTo],row_number() over(partition by b.courseid order by b.ActStartDate desc) as rn
from [SQL3].[dbo].[Training_Record] a inner join 

(select z.schid, z.CourseID as [CourseID],max(z.ActStartDate) as [ActStartDate]
from [SQL3].[dbo].[Training_Schedule] z group by z.SchID, z.CourseID)as b

    on a.SchID = b.SchID 

-- to test data content
where EmpNo = '141281' and CourseID = '22'
)A where rn=1