纵向合并排除的问题

想把数据集a与数据集b纵向合并，想把a中按照 id year ym存在的观测   ，如果b中也具有三者取值相同的观测，那么就用a中的这些观测覆盖掉b中的这些观测。
谢谢

data final;
  set a b;
run;

proc sort data=final nodupkey;
  by id year ym;
run;

assume you want to keep the original B, just use the variable in A file to replace

proc sql;
  create table new as select distinct b.id, b.year, b.ym, a.*
  from B as a left join A as b
  on a.id=b.id & a.year=b.year, a.ym=b.ym;
quit;