insert into z1(ls,ds,dpo,x)
values( 1 ,'2006-06-01','2006-06-30', 20 );
insert into z1(ls,ds,dpo,x)
values( 1 ,'2006-07-01','2006-12-31', 10 );


create VIEW "DBA"."z3" as /* view_column_name, ... */
  /* Type the select statement here */
  select z1.ls,z1.ds,z1.dpo,z1.x,null as y
    from DBA.z1 union all
  select z2.ls,z2.ds,z2.dpo,null as x,z2.y
    from DBA.z2

select z.ls,z.ds,
(select first z1.dpo from z3 as z1 where z.ls = z1.ls
and isnull(z.x, 0 )=isnull(z1.x, 0 )
and z.ds < z1.dpo 
and not exists(select * from z3 as z3 where z1.ls=z3.ls and isnull(z1.x, 0 )=isnull(z3.x, 0 ) and z3.ds = z1.dpo+ 1 )
) as dpo,
isnull(z.x,(select first z1.x from z3 as z1 where z.ls = z1.ls
and z1.dpo+ 1  <= z.ds and z1.x is not null order by z1.ds desc
)) as x,
isnull(z.y,(select first z1.y from z3 as z1 where z.ls = z1.ls
and z1.dpo+ 1  <= z.ds and z1.y is not null order by z1.ds desc
)) as y
from z3 as z
where not exists(select * from z3 as z2 where z.ls = z2.ls
and isnull(z.x, 0 ) = isnull(z2.x, 0 ) and isnull(z.y,'') = isnull(z2.y,'')
and z.ds = z2.dpo+ 1 ) 
order by z.ds