sql - 为另一个表中的每个 ID 从表中选择前 1 个

Question

表结构是:

create table fruit (
  id int identity(1,1),
  name varchar(max)
)

create table fruit_allocation (
  id int identity(1,1),
  fruit_id int references fruit(id),
  customer_id int references store(id),
  amount float,
)

create table measurement (
  fruit_allocation_id int references fruit_allocation(id),
  measurement_date datetime,
  measurement float,
)

每个水果可以分配给多个客户，创建一个 Fruit_allocation 记录。每个fruit_allocation 记录可以有多个度量。

我想为每个给定水果 ID 的水果分配选择最新的测量值

到目前为止，我有以下几点:

select * 
  from measurement 
 where fruit_allocation_id in (select id 
                                 from fruit_allocation 
                                where fruit_id = 10)

这将返回该水果的所有测量值，我只想为每个水果分配返回 1 个测量值。

Answer 1

您可以 CROSS APPLY

select a.*, m.*
from fruit_allocation a
cross apply (
    select top 1 *
    from measurement m
    where m.fruit_allocation_id = a.id
    order by m.measurement_date desc
) m
where a.fruit_id = 10