proc sql;
create table wanted as
select b.*, a.x
from
(select *
from (select distinct id from aaa), (select distinct visit from bbb)) b
left join aaa a
on a.id=b.id and a.visit=b.visit;
quit;

data步可以实现吗？

如何得到这样的结果
1 a 1 1
1 a 2 2
1 a 3 .
1 b 1 3
1 b 2 4
1 b 3 5
2 a 1 1
2 a 2 .
2 a 3 .
2 b 1 3
2 b 2 4
2 b 3 .