declare @表A table (ID int,A1 varchar(2))
insert into @表A
select 1,'a1' union all
select 2,'a2' union all
select 3,'a3'

declare @表B table (ID int,B1 varchar(2))
insert into @表B
select 3,'b3' union all
select 4,'b4' union all
select 5,'b5'

select isnull(a.ID,b.ID) as ID,a.A1,b.B1 from @表A a 
full join @表B b on a.ID=b.ID

/*
ID          A1   B1
----------- ---- ----
1           a1   NULL
2           a2   NULL
3           a3   b3
4           NULL b4
5           NULL b5
*/