Oracle子查询

Question

我有2个表PORT和发货。我必须选择对应于最大（计数）的名称。它会为别名“A”引发错误。这段代码或其他可用的替代品是否有错误？

select name from 
    (
    select name, count(name) as countval from 
    (
    select p.name from port p 
    inner join shipment s on p.id =s.departure_port_id 
    ) 
    group by name 
    order by countval desc 
     ) A 
     where countval=(select max(countval) from A)); 

Answer 1

您可以使用rank窗口功能。如果有最大数量的关系，结果会给你多个名字。

select name from (
select p.name,rank() over(order by count(*) desc) as rnk 
from port p 
join shipment s on p.id =s.departure_port_id 
group by p.name 
) t 
where rnk=1 

Answer 2

如果你想使用具有阻燃过滤结果，你可以使用

select name from (
    select p.name, count(p.name) my_count from port p 
    inner join shipment s on p.id =s.departure_port_id 
    group by p.name) t 
having t.my_count = max(t.my_count) 
group by name 

Answer 3

你想与大多数出货端口的端口名称？这可以在几个关系的情况下。一种方法是汇总运输表以获取这些端口号并使用这些端口号来选择端口名称。这是甲骨文12C：

select name 
from port 
where id in 
(
    select departure_port_id 
    from shipment 
    group by departure_port_id 
    order by count(*) desc 
    fetch first 1 row with ties 
); 

在早期版本中，你将使用RANK而不是排名侑装运聚集，这是略少可读性。例如：

select name 
from port 
where (id, 1) in 
(
    select departure_port_id, rank() over (order by count(*) desc) 
    from shipment 
    group by departure_port_id 
);