用retain就可以做了

data a;
   input a;
cards;
1
2
3
4
;
run;

proc sql;
       create table b as
       select a.a, sum(b.a) as b
       from   a as a, a as b
       where  a.a>=b.a
       group by a.a
       ;
quit;

this is not a general solution. what if the series not sorted ascendingly?

模仿以下，既然位置更重要(but my code looks very stupid because of the funny sql function)。
京剧

SQL will be a clumsy approach for solving this problem. There is no observation order concept in database or SQL language. For example, find the third largest value of a variable.  

The proposed solution would be wrong in following data ordering,

data a;
   input a;
cards;
1
6
3
4
;
run;

data b;
   set a;
   b+a;
run;

proc print;run;

proc sql;
      
       select a.a, sum(b.a) as b
       from   a as a, a as b
       where  a.a>=b.a
       group by a.a
       ;
quit;