SQL - Calculate Incentive using Frequency (Mode) - sql

Question

I’d like a SQL query to calculate the incentive column and based on the number of times a student was tutored in a month.
If the student was tutored in AP Math twice in a month then, the Tutor gets $20, else $0.
I don’t want group the date into month summary so, I’d like to leave the dates as is hence why I assigned $10 for each record where the students that were tutored twice in month.

Answer 1

I think you just need window functions:
select t.*,
(case when cnt = 2 then 10 else 0 end) as incentive
from (select t.*,
count(*) over (partition by studentid, subject, tutor, year(studentvisitdate), month(studentvisitdate) as cnt
from t
) t

Answer 2

initializing the data
declare #StudentTable as table
(
StudentVisitDate date,
StudentId int,
StudentName varchar(100),
[Subject] varchar (30),
Tutor varchar(100),
Incentive int
)
insert into #StudentTable
values
('2018-August-03',123,'Terry Brooks','AP Math','Shawn Green',10)
,('2018-August-04',123,'Terry Brooks','AP Science','Ted Berry',10)
,('2018-August-07',123,'Terry Brooks','Music','Shawn Green',10)
,('2018-September-03',123,'Terry Brooks','AP Math','Shawn Green',10)
,('2018-September-04',123,'Terry Brooks','AP Science','Ted Berry',10)
,('2018-September-07',123,'Terry Brooks','AP Math','Shawn Green',10)
getting the count of visits per month, grouping the data
;with temp as
(
select
StudentId, Subject, Tutor,Month(StudentVisitDate) [month]
,max(StudentVisitDate) maxdate -- the date on which the incentive will be calculated
,Count(StudentVisitDate) [count]
from #StudentTable
Group by
StudentId, Subject, Tutor,Month(StudentVisitDate)
)
getting the effective incentives for each month using the information from above table
select
s.StudentVisitDate ,
s.StudentId ,
s.StudentName ,
s.[Subject] ,
s.Tutor ,
Case
when t.maxdate = s.StudentVisitDate -- the incentive will be applied on the maximum date
then
s.Incentive*t.count
else
0
end Incentive
from #StudentTable s
inner join temp t
on s.StudentId=t.StudentId
and s.Subject =t.Subject
and s.Tutor=t.Tutor
and Month(s.StudentVisitDate)=t.month
order by StudentVisitDate
Final output -
StudentVisitDate StudentId StudentName Subject Tutor Incentive
2018-08-03 123 Terry Brooks AP Math Shawn Green 10
2018-08-04 123 Terry Brooks AP Science Ted Berry 10
2018-08-07 123 Terry Brooks Music Shawn Green 10
2018-09-03 123 Terry Brooks AP Math Shawn Green 0
2018-09-04 123 Terry Brooks AP Science Ted Berry 10
2018-09-07 123 Terry Brooks AP Math Shawn Green 20