我写了下面的查询,它给出了结果集 -
select
a.character_name,
b.planet_name,
sum(b.departure - b.arrival) AS screen_time
from characters a
inner join timetable b
on a.character_name = b.character_name
group by a.character_name, b.planet_name;
结果集:
Character_name | planet_name | Screen_time
C-3 PO Bespin 4
C-3 PO Hoth 2
C-3 PO Tatooine 4
Chewbacca Bespin 4
Chewbacca Endor 5
Chewbacca Hoth 2
Chewbacca Tatooine 4
现在,如何选择具有max(screen_time)的每个字符的planet_name和character_name。 对于前者, 对于C-3 PO,要显示两行
C-3 PO | Bespin
C-3 PO | Tattoine
对于Chewbacca,要显示一行
Chewbacca | Endor
我面临的问题是因为我无法将条件实施到中间表。
答案 0 :(得分:1)
编辑(删除我之前的回答)
您也可以使用HAVING
子句中的相关子查询实现相同的目标:
select
a.character_name,
b.planet_name,
sum(b.departure - b.arrival) as screen_time
from characters a
inner join timetable b
on a.character_name = b.character_name
group by a.character_name, b.planet_name
having sum(b.departure - b.arrival) = (
select
max(tmp.screen_time)
from (
select b.character_name, sum(b.departure - b.arrival) as screen_time
from timetable b
group by b.character_name, b.planet_name) tmp
where a.character_name = tmp.character_name
group by tmp.character_name);
鉴于您的问题,我不确定您是要检索screen_time
还是character_name
和planet_name
。如果您只想要最后两列,请从主查询中删除sum(b.departure - b.arrival) as screen_time
:
select
a.character_name,
b.planet_name
from characters a
inner join timetable b
on a.character_name = b.character_name
group by a.character_name, b.planet_name
having sum(b.departure - b.arrival) = (
select
max(tmp.screen_time)
from (
select b.character_name, sum(b.departure - b.arrival) as screen_time
from timetable b
group by b.character_name, b.planet_name) tmp
where a.character_name = tmp.character_name
group by tmp.character_name);
PS:我以前的答案没有用。对不起。
答案 1 :(得分:1)
select character_name, planet_name
from (
select
a.character_name,
b.planet_name,
sum(b.departure - b.arrival) AS screen_time
from characters a
inner join timetable b
on a.character_name = b.character_name
group by a.character_name, b.planet_name
) sq
where screen_time = (SELECT MAX(ssq.screen_time) FROM (
select
a.character_name,
b.planet_name,
sum(b.departure - b.arrival) AS screen_time
from characters a
inner join timetable b
on a.character_name = b.character_name
group by a.character_name, b.planet_name
) ssq WHERE ssq.character_name = sq.character_name
);
但不要担心,性能不应该像它最初看起来那么糟糕。 MySQL通常足够聪明,不能两次执行相同的查询。
还有其他方法可以实现同样的目标。如果您愿意,也可以尝试这些:The Rows Holding the Group-wise Maximum of a Certain Column