update tbl1 t1 set b=
(select b from tbl1 t2 where t1.b is not null and b<>'' and t1.a=t2.a where rownum<2)
where b is null or b=''
(select b from tbl1 t2 where t1.b is not null and b<>'' and t1.a=t2.a where rownum<2)
where b is null or b=''
调试欢乐多
怎么又两个where
where exists(select 1 from tbname tt where tt.a=t.a);