mysql - Mysql双聚合函数

Question

我想从一系列返回值中获取最大值，但我无法找到一个简单的方法来做到这一点。我的查询返回所有行，所以 1/2 在那里。我可以用 PHP 过滤它，但我想用 SQL 来完成这一切。我尝试使用 max子查询，但仍然返回所有结果。

DDL:

create table matrix(
   count int(4), 
   date date, 
   product int(4)
);
create table products(
   id int(4), 
   section int(4)
);

DML:

select max(magic_count), section, id
from (
    select sum(count) as magic_count, p.section, p.id
    from matrix as m
    join products as p on m.product = p.id
    group by m.product
) as faketable
group by id, section

Demo with my current try.

仅 ID 1和3应该从样本数据中返回，因为它们具有最高的累积 count对于每个 section s。

Here's a second SQL fiddle that demonstrates the same issue.

Answer 1

给你:

select a.id, 
       a.section,
       a.magic_count
from (
    select p.id,
           p.section,
           magic_count
    from (
        select m.product, sum(count) as magic_count
        from matrix m
        group by m.product
    ) sm
    join products p on sm.product = p.id
) a
left join (
    select p.id,
           p.section,
           magic_count
    from (
        select m.product, sum(count) as magic_count
        from matrix m
        group by m.product
    ) sm
    join products p on sm.product = p.id
) b on a.section = b.section and a.magic_count < b.magic_count
where b.id is null

• 请参阅 The Rows Holding the Group-wise Maximum of a Certain Column 的手册条目中的简化示例(和其他方法)

• 查看它的实时运行 here