SELECT a single field by ordered value - sql

Question

Consider the following two tables:
student_id score date
-------------------------
1 10 05-01-2013
2 100 05-15-2013
2 60 05-01-2012
2 95 05-14-2013
3 15 05-01-2011
3 40 05-01-2012
class_id student_id
----------------------------
1 1
1 2
2 3
I want to get unique class_ids where the score is above a certain threshold for at least one student, ordered by the latest score.
So for instance, if I wanted to get a list of classes where the score was > 80, i would get class_id 1 as a result, since student 2's latest score was above > 80.
How would I go about this in t-sql?

Answer 1

Are you asking for this?
SELECT DISTINCT
t2.[class_ID]
FROM
t1
JOIN t2
ON t2.[student_id] = t1.[student_id]
WHERE
t1.[score] > 80

Answer 2

Edit based on your date requirement, then you could use row_number() to get the result:
select c.class_id
from class_student c
inner join
(
select student_id,
score,
date,
row_number() over(partition by student_id order by date desc) rn
from student_score
) s
on c.student_id = s.student_id
where s.rn = 1
and s.score >80;
See SQL Fiddle with Demo
Or you can use a WHERE EXISTS:
select c.class_id
from class_student c
where exists (select 1
from student_score s
where c.student_id = s.student_id
and s.score > 80
and s.[date] = (select max(date)
from student_score s1
where s.student_id = s1.student_id));
See SQL Fiddle with Demo

Answer 3

select distinct(class_id) from table2 where student_id in
(select distinct(student_id) from table1 where score > thresholdScore)

Answer 4

This should do the trick:
SELECT DISTINCT
CS.Class_ID
FROM
dbo.ClassStudent CS
CROSS APPLY (
SELECT TOP 1 *
FROM dbo.StudentScore S
WHERE CS.Student_ID = S.Student_ID
ORDER BY S.Date DESC
) L
WHERE
L.Score > 80
;
And here's another way:
WITH LastScore AS (
SELECT TOP 1 WITH TIES
FROM dbo.StudentScore
ORDER BY Row_Number() OVER (PARTITION BY Student_ID ORDER BY Date DESC)
)
SELECT DISTINCT
CS.Class_ID
FROM
dbo.ClassStudent CS
WHERE
EXISTS (
SELECT *
FROM LastScore L
WHERE
CS.Student_ID = L.Student_ID
AND L.Score > 80
)
;
Depending on the data and the indexes, these two queries could have very different performance characteristics. It is worth trying several to see if one stands out as superior to the others.
It seems like there could be some version of the query where the engine would stop looking as soon as it finds just one student with the requisite score, but I am not sure at this moment how to accomplish that.