How to output a value refer to another value? - sql

Question

I have a table which including Student's name, course_id and score, the different students may have a different course and different score.
And now I need to output the academic standings for each student, here is the rule for the academic standings:
If a student takes only one course(count(course_id)=1), he/she will receive ‘Good’ if the score >= 50, and ‘Referral’ if the score < 50;
If a student takes more than one course (count(course_id)>=2), his/her academic standing would be ‘Probation’ if none of the score >=50, ‘Referral’ if 50% or less of the taken courses are score >=50, and ‘Good’ otherwise.
Table:
Student_name| course_id |score
Shirley Caws 55993 10
Lana Glendenning 56988 81
Michael 54880 80
Michael 54895 71
Sean Turle 56986 32
Sean Turle 56991 48
Sean Turle 56992 20
Damchu Tenzin 56215 40
Damchu Tenzin 56219 90
Blake Croll 57179 30
Blake Croll 57264 20
I have tried for an hour in writing "CASE WHEN" but fail to get the right answer.
SELECT student_name, (CASE WHEN .... THEN ELSE END) AS academic standings FROM table;
Expected results:
Student_name| Academic_standings
Shirley Caws Referral
Lana Glendenning Good
Michael Good
Sean Turle Probation
Damchu Tenzin Referral
Blake Croll Probation
Thanks!

Answer 1

I'd try something like this - count the classes first, then apply your logic:
; with CTE as (
Select StudentName
, Count(distinct CourseID) as TotalClasses
, Count(distinct case when score < 50 then CourseID end) as ClassesUnder50
From MyTable
Group By StudentName
)
Select StudentName
, Case when TotalClasses = 1 and ClassesUnder50 = 0 then 'Good'
when TotalClasses = 1 and ClassesUnder50 = 1 then 'Referral'
when TotalClasses > 1 and ClassesUnder50 = TotalClasses then 'Probation'
when TotalClasses > 1 and ClassesUnder50*1.0/TotalClasses >= 0.5 then 'Referral'
else 'Good' end as Standing
from CTE

Answer 2

You can use aggregation functions and conditional aggregation:
select student_name,
(case when count(*) = 1 and max(score) >= 50 then 'Good'
when count(*) = 1 then 'Referral'
when max(score) < 50 then 'Probation'
when sum(case when score >= 50 then 1 else -1 end) <= 0 then 'Referral'
else 'Good'
end) as academic_standing
from t
group by student_name;