SQL MAX 聚合函数没有带来最新日期

Question

目的：我试图找到老师购买和打字的最大日期。

订单table

ID	Ordertype	Status	TeacherID	PurchaseDate	SchoolID	TeacherassistantID
1	Pencils	Completed	1	1/1/2021	1	1
2	Paper	Completed	1	3/5/2021	1	1
3	Notebooks	Completed	1	4/1/2021	1	1
4	Erasers	Completed	2	2/1/2021	2	2

教师table

TeacherID	Teachername
1	Mary Smith
2	Jason Crane

学校table

ID	schoolname
1	ABC school
2	PS1
3	PS2

这是我尝试的代码：

SELECT o.ordertype, o.status, t.Teachername, s.schoolname
,MAX(o.Purchasedate) OVER (PARTITION by t.ID) last_purchase
FROM orders o
INNER JOIN teachers t ON t.ID=o.TeacherID
INNER JOIN schools s ON s.ID=o.schoolID
WHERE o.status in ('Completed','In-progress')
AND o.ordertype not like 'notebook'

它应该是这样的：

Ordertype	Status	teachername	last_purchase	schoolname
Paper	Completed	Mary Smith	3/5/2021	ABC School
Erasers	Completed	PS1	2/1/2021	ABC school

它带来了多行，而不仅仅是最新购买日期及其关联的行。我想我需要一个子查询。

Answer 1

聚合函数不适合您要执行的操作。它们的目的是汇总多行中的值，而不是选择特定的行。

只是一个 window 函数不会过滤任何行。

您想使用 window 函数进行过滤：

SELECT ordertype, status, Teachername, schoolname, Purchasedate
FROM (SELECT o.ordertype, o.status, t.Teachername, s.schoolname,
             o.Purchasedate,
           ROW_NUMBER() OVER (PARTITION by t.ID ORDER BY o.PurchaseDate DESC) as seqnum
      FROM orders o JOIN
           teachers t
           ON t.ID = o.TeacherID
           schools s
           ON s.ID = o.schoolID
      WHERE o.status in ('Completed', 'In-progress') AND
            o.ordertype not like 'notebook'
     ) o
WHERE seqnum = 1;

Answer 2

你可以用不同的方式使用它。最好使用 Group By 对其他列进行分组，然后使用 Order by 对所有记录重新排序，就像下面一样。

SELECT top 1 o.ordertype, o.status, t.Teachername, s.schoolname
,o.Purchasedate 
FROM orders o
INNER JOIN teachers t ON t.ID=o.TeacherID
INNER JOIN schools s ON s.ID=o.schoolID
having o.status in ('Completed','In-progress')
AND o.ordertype not like 'notebook'
group by o.ordertype, o.status, t.Teachername, s.schoolname
order by o.Purchasedate  Desc

SQL MAX 聚合函数没有带来最新日期

SQL MAX aggregate function not bringing the latest date

sql

subquery

aggregate-functions