SQL两表合并（两表列相同）

select a.* from
    (select distinct htno,c_value,row_number()over(partition by htno order by samply_no desc) rn from BGTAMODLL3.T_ODS_TQMTJ04 where rec_creator='100' and c_value>0 and c_value<20) a
where a.rn='1'

union all

select * from
    (select distinct htno,c_value as dc_c,row_number()over(partition by htno order by sample_time desc) rn from BGTAMODLL3.T_ODS_TMMSM35  where htno not in (select distinct htno from a) and c_value>0 and c_value<1)
where rn='1'
order by htno

问题出在第二段程序 htno not in (select distinct htno from a) 这个地方，这个去掉程序可以实现，主要是第二个程序的htno和第一段是相关的，这种情况要怎么处理才能让两个表并在一起。