update scores as s1
   set score=exptbl.exp_score
  from (select s.test_code, 
               s.student_code, 
               s.subject_code, 
               s.score, 
               FLOOR(s.score/0.9) as exp_score 
          from scores s inner join subjects on s.subject_code=subjects.subject_code 
         where subject_name='国語'
       ) 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
;