update scores as s1
   set score=CASE exptbl.exp_score WHEN exptbl.exp_score > 100 THEN 100 ELSE exptbl.exp_score END
  from (select s.test_code, 
               s.student_code, 
               s.subject_code, 
               s.score, 
               s.score+5 as exp_score 
          from scores s inner join students on s.student_code=students.student_code 
         where student_name in ('佐藤 幸平', '山田 康介')
       ) as exptbl
where s1.test_code=exptbl.test_code
  and s1.student_code=exptbl.student_code
  and s1.subject_code=exptbl.subject_code
;
select * from scores
;