data  a;
input a $ b;
cards;
a 1
b 2
c 3
d 4
;
run;
proc sql;
  create table b as
  select *, b/sum(b) as pct format=percent8.
  from a;
  quit;

如果是在同一个DATA步里实现的话该怎么写程序？谢谢！

But you will have to read twice, one for calculating a total and the other for calculating a percent.

Here it is. This sort of programming style  is posted several times.There are some good knowledge points here.

data  a;
input a $ b;
cards;
a 1
b 2
c 3
d 4
;
run;

data b;
   do _n_=1 to nobs;
       set a nobs=nobs;
       tot_b+b;
   end;
   do _n_=1 to nobs;
       set a ;
       pct=b/tot_b;
       output;
   end;
run;

proc print; run;

proc sql;
  select *, b/sum(b) as pct
  from a;
  quit;

学习了