Using CASE WHEN To Update - sql

Question

I have a few tables that look like
select * from stipend
order by subjectid, stipdate;
SUBJECTID STIPDATE AMOUNT
---------- ----------- ----------
10011 31-oct-2021 800
10111 31-jul-2019 2000
10111 31-jul-2021 1500
20022 31-jul-2020 1200
30033 29-feb-2020 1400
40044. 31-jul-2020 1200
40044 31-jul-2021 2000
50055 31-jul-2021 2000
50055 30-sep-2021 1000
select * from subject
order by subjectid;
SUBJECTID LNAME FNAME PROJID
---------- ------------ --------------------
10011 Indy Eva XYZ01
10111 Isner Monica XYZ04
11011 Dupont Marty XYZ05
20022 Jordan Sam XYZ01
30033 Jordan Mary XYZ01
40044 Belmont Renee XYZ02
50055 Pissaro Becky XYZ02
60066 Nadal Becky XYZ03
70077 Bardot Brigitte XYZ03
80088 null Eva XYZ03
90099 Garnet Larry XYZ04
And I want to update the stipends that are in Project XYZ01 and XYZ02 by 40% using the CASE WHEN construct. Obviously the two tables are connect by stipend.subjectid = subject.subjectid and the I would use the subject.projectid to determine this, but how would I do that with CASE WHEN?
This is how I did it before
update stipend
set amount = amount + (amount * .40)
where subjectid in
(select subjectid from subject
where subject.projid = 'XYZ01' or subject.projid = 'XYZ02');
and that worked, but now I need to do the same thing but with CASE WHEN.

Answer 1

Eddie, below is a solution that uses the CASE WHEN statement. But, as Mureinik mentioned on the comments, your solution is better than using a CASE WHEN statement in this case.
-- using CASE WHEN
update s
set s.amount = (case when x.projid in ('XYZ01','XYZ02')
then s.amount + (s.amount * .40)
else s.amount
end)
from stipend s
join subject x on x.subjectid = s.subjectid
Note: I did not add x.projid in ('XYZ01','XYZ02') to the where clause as that would defeat the purpose of using the CASE WHEN. So, this would run for all the rows.