data raw;
input year month;
cards;
1998 9
1998 9
1998 10
1998 10
1999 5
1999 5
1999 5
1999 6
1999 6
;
run;
proc sort data=raw;
by year month;
run;

data a;
set raw;
by year month;
retain id;
if  first.month then id=1;
else id+1;
run;

data result;
   set a;
   retain type 0;
   if id=1 then do;
     type+1;
  end;
  else;
  drop id;
  rename type=id;
run;

data ym;
   input year moth @@;
   cards;
1998 9
1998 9
1998 10
1998 10
1999 5
1999 5
1999 5
1999 6
1999 6
;
proc sort data=ym;
   by year month;
   run;
data want;
  set ym;
  by year month;
  retain id o;
  if  first.month  then do;
  id+1;
  end;
run;

[此贴子已经被作者于2009-3-27 18:59:58编辑过]

谢谢楼上的两位，我试了下，两种方法都可以！！

学习了！

谢谢！