Select Top 1 From a Table For Each Id in another Table

Question

Table Structure is:

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,
)

Each fruit can be allocated to more than one customer creating a fruit_allocation record. Each fruit_allocation record can have multiple measurements.

I want to select the latest measurement for each fruit_allocation given a fruit id

So far I have the following:

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

This returns all measurements for that fruit, I want to just return 1 measurement per fruit_allocation.

Answer 1

You can 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

Answer 2

Assuming you are using SQL Server 2005+

With RankedMeasurements As
    (
    Select M.fruit_allocation_id
        , M.measurement_date
        , M.measurement
        , Row_Number() Over ( Partition By M.fruit_allocation_id 
                                Order By M.measurement_date Desc ) As Rnk
    From measurement As M
    Where Exists    (
                    Select 1
                    From fruit_allocation As FA1
                    Where FA1.id = M.fruit_allocation_id
                        And FA1.fruit_id = 10
                    )   
    )
Select RM.fruit_allocation_id
    , RM.measurement_date
    , RM.measurement
From RankedMeasurements As RM
Where Rnk = 1