类似于我的代码，就是之前给个自增id就可以了

SQL 程序可以加一个自增变量：

PROC sql;
   create table b as
      select *,monotonic() as n
        from a;
quit;

之后步骤见2楼。

data test;
   input X1 $ X2;
   cards;
01 10
01 .
01 .
01 20
01 .
02 100
02 .
02 120
02 .
02 140
02 .
;


proc sql;
    create table test1 as
          select x1, x2, monotonic() as n
          from test
      ;

   create table test2(keep=x1 x2) as
          select distinct l.x1,l.n,r.x2
             from test1 as l,
                          (select distinct x2,n as group from test1 where ^missing(x2))as r
       where l.n>=group
        group by n
        having group=max(group)
        order by n;
quit;

This query can solve your question 2).

data a;
input X1 $  X2;
if ^missing(x2) then flag+1;
cards;
01 10
01 .
01 .
01 20
01 .
02 100
02 .
02 120
02 .
02 140
02 .
;
proc sql;
   create table b as
     select t.x1,s.x2
         from a t
   left join
   (select x1, x2, flag
          from a
          where x2 ne . ) s
   on t.x1=s.x1 and t.flag=s.flag
         ;
quit;
proc print;run;