非空值优先于空的不同行

Question

我有两行重复两行，但该行的一列具有空值，另一列具有某个值。例如

CarName  Owner  PreviousOwner
Honda    XXX    YYYYY
Honda    XXX    
Nissan   ZZZ    AAAA
Nissan   ZZZ    
BMW      BBB
Benz     CCC    DDD

输出应该是

Honda    XXX    YYYYY
Nissan   ZZZ    AAAA
BMW      BBB
Benz     CCC    DDD

能帮我写一个对此的查询吗？

Answer 1

检查

select CarName, Owner, max(PreviousOwner)
from t
group by CarName, Owner

Answer 2

如果我们要对记录进行排序，以使前一个拥有值的所有者来    在最上面，那么您要做的就是选择最上面的行。为了实现这一点，我使用了row_number（）以及previous_owner的顺序。请看下面：

select carname, owner, previous_owner from 
(select cars.*, rowid, row_number() over (partition by carname, owner order by 
previous_owner) rid 
from cars) 
where rid <=1;

Answer 3

一种方法是：

select t.CarName, t.Owner, r.PreviousOwner
from (select Carname, max(previousowner)
       from test 
       group by carname) r
inner join test t 
on t.carname = r.carname and t.previousowner = r.previousowner
group by t.CarName, t.Owner, r.PreviousOwner;