proc sort data=a; by id; quit;

data b; set a;
by id;
retain seq;
if first.id then seq=0;
seq = seq + 1;
run;

感谢你

data test;
set sashelp.class;
run;
proc sort data=test;
by age;
run;
data test;
set test;
by age;*若把此句删除就出不来结果；
retain seq;
if first.age then seq=0;
seq=seq+1;
run;

此处的By语句作用是啥？好神奇！！

set+by语句是一个组合