data raw;
  input id $ x y;
  datalines;
A  1  2
A  2  3
B  1  4
B  2  5
  ;
run;

普通方法：
proc sql;
  create table result1 as
    select raw.*,       
           mean(x) as x_mean,
           mean(y) as y_mean
          from raw
            group by id;
quit;
proc print; run;

文艺方法：
proc summary data=raw nway;
  class id;
  var x y;
  output out=raw_smy(drop=_:) mean=/autoname;
run;
data result2;     
  retain id x y x_mean y_mean;
  if _n_=1 then do;
    if 0 then set raw_smy;
    declare hash
      ht(dataset: "raw_smy");
      ht.definekey("id");
      ht.definedata(all: "yes");
      ht.definedone();
  end;
  set raw;
  ht.find(key: id);
run;
proc print; run;

2B方法：
data result3;  
  if _n_=1 then do until (a_obs_last);   
    set raw(where=(id="A")) end=a_obs_last;   
    a_n+1;
    a_x_sum+x;  
    a_y_sum+y;
  end;
  if _n_=1 then do until (b_obs_last);
    set raw(where=(id="B")) end=b_obs_last;   
    b_n+1;
    b_x_sum+x;   
    b_y_sum+y;
  end;  
  set raw;  
  if id="A" then do;
    x_mean=a_x_sum/a_n;
    y_mean=a_y_sum/a_n;
  end;
  else if id="B" then do;
    x_mean=b_x_sum/b_n;
    y_mean=b_y_sum/b_n;
  end;
keep id x y x_mean y_mean;
run;
proc print; run;

感谢二楼~