问SQL还是UNION还是什么？

EN

select a.a, b.b, c.c
from (select a.*, row_number() over (order by a) as seqnum
      from a
     ) a full outer join
     (select b.*, row_number() over (partition by a order by b) as seqnum
      from b
     ) b
     on a.a = b.a and a.seqnum = b.seqnum full outer join
     (select c.*, row_number() over (partition by a order by c) as seqnum
      from c
     ) c
     on c.a = coalesce(a.a, b.a) and c.seqnum = coalesce(a.seqnum, b.seqnum)
where coalesce(a.a, b.a, c.a) = 1;