Find the top value for each parent - sql

Question

I'm sure this is a common request but I wouldn't know how to ask for it formally.
I encountered this a long time ago when I was in the Army. A soldier has multiple physical fitness tests but the primary test that counts in the most recent. The soldier also has multiple marksmanship qualifications but only the most recent qualification to the weapon assigned is significant.
How do you create a view that itemizes the most significant child of the parent?

Answer 1

Use:
SELECT p.*, x.*
FROM PARENT p
JOIN CHILD x ON x.parent_id = p.id
JOIN (SELECT c.id,
c.parent_id,
MAX(c.date_column) AS max_date
FROM CHILD c
GROUP BY c.id, c.parent_id) y ON y.id = x.id
AND y.parent_id = x.parent_id
AND y.max_date = x.date
Assuming SQL Server 2005+:
WITH summary AS (
SELECT p.*,
c.*,
ROW_NUMBER() OVER (PARTITION BY p.id
ORDER BY c.date DESC) AS rank
FROM PARENT p
JOIN CHILD c ON c.parent_id = p.id)
SELECT s.*
FROM summary s
WHERE s.rank = 1

Answer 2

Although I'm not quite sure what you are implying by "itemizing", you can do something like so:
Select ..
From Soldier
Left Join FitnessTest
On FitnessTest.SoldierId = Soldier.Id
And FitnessTest.TestDate = (
Select Max(FT1.TestDate)
From FitnessTest As FT1
Where FT1.SoldierId = FitnessTest.SoldierId
)
Left Join MarksmanshipTest
On MarksmanshipTest.SoldierId = Soldier.Id
And MarksmanshipTest.TestDate = (
Select Max(MT1.TestDate)
From MarksmanshipTest As MT1
Where MT1.SoldierId = MarksmanshipTest.SoldierId
)
This assumes that a solider cannot have two test datetime values for a fitness test or a marksmanship test.

Answer 3

No significant differnce from previous two answer but a little more detail perhaps:
create table soldier ( soldierId int primary key,
name varchar(100) )
create table fitnessTest ( soldierId int foreign key references soldier,
occurred datetime, result int )
create table marksmanshipTest ( soldierId int foreign key references soldier,
occurred datetime, result int )
;with
mostRecentFitnessTest as
(
select
fitnessTest.soldierId,
fitnessTest.result,
row_number() over (order by occurred desc) as row
from fitnessTest
),
mostRecentMarksmanshipTest as
(
select
marksmanshipTest.soldierId,
marksmanshipTest.result,
row_number() over (order by occurred desc) as row
from marksmanshipTest
)
select
soldier.soldierId,
soldier.name,
mostRecentFitnessTest.result,
mostRecentMarksmanshipTest.result
from soldier
left outer join mostRecentFitnessTest on
mostRecentFitnessTest.soldierId = soldier.soldierId
and mostRecentFitnessTest.row = 1
left outer join mostRecentMarksmanshipTest on
mostRecentMarksmanshipTest.soldierId = soldier.soldierId
and mostRecentMarksmanshipTest.row = 1