data test1;
  input a b c;
  cards;
1   4   .
1   5   .
1   7   7
1   8   .
1   9   9
2   4   4
2   5   .
2   6   6
2   7   .
3   4   .
3   5   5
3   6   .
3   7   7
;
proc sort; by a b c; run;
data test2;
  set test1;
  by a;
  retain seq 1;
  if ~missing(c) then do; seq + 1; end;
run;
proc sql noprint;
   create table test3 as
   select count(seq) as num, seq, a, b, c from test2
   group by seq;
quit;
data test4;
  set test3;
  where seq ^=1 and num=2 and missing(c);
  c = b;
run;
data test_final;
  set test4 test3(where = (num^=2)) test3(where=(num=2 and seq =1)) test3(where = (num=2 and ~missing(c)));
  drop seq num;
run;
proc sort; by a b c; run;
用这个没有d

data test1;
  input a b c;
  cards;
1   4   .
1   5   .
1   7   7
1   8   .
1   9   9
2   4   4
2   5   .
2   6   6
2   7   .
3   4   .
3   5   5
3   6   .
3   7   7
;
proc sort; by a b c; run;
data test2;
  set test1;
  by a;
  retain seq 1;
  if ~missing(c) then do; seq + 1; end;
run;
proc sql noprint;
   create table test3 as
   select count(seq) as num, seq, a, b, c from test2
   group by seq;
quit;
data test4;
  set test3;
  where seq ^=1 and num=2 and missing(c);
  c = b;
run;
data test_final;
  set test4 test3(where = (num^=2)) test3(where=(num=2 and seq =1)) test3(where = (num=2 and ~missing(c)));
  drop seq num d;
run;
proc sort; by a b c; run;
看下能不能再简化一下~

data obs1;
set test;
obs=_n_;
run;

proc sort data=obs1;
by a obs;
run;

proc sql;
create table obs2 as
select a, min(obs) as minobs, max(obs) as maxobs
from obs1
where c ne .
group by a
order by a;
quit;

data wanted;
merge obs1 obs2;
by a;
if minobs<obs<maxobs then c=b;
drop obs minobs maxobs;
run;