mysql - 对多列进行分组

Question

我有以下查询:

SELECT od.product_id, od.product_name, od.product_quantity, od.product_price,
        o.id_address_delivery, a.id_country, a.id_state, s.name
FROM `ps_order_detail` od
LEFT JOIN ps_orders AS o ON od.`id_order` = o.`id_order`
LEFT JOIN `ps_address` AS a ON o.`id_address_delivery` = a.`id_address`
LEFT JOIN `ps_state` AS s ON a.id_state = s.id_state
ORDER BY o.`date_add` DESC
LIMIT 100;

它返回如下数据:

产品 ID |产品名称 |产品数量 |产品价格 | id_state

我想知道每个州的畅销书(就数量而言)。例如:

状态 A:最畅销的是产品 A(已售出 500 件) 状态 B:最畅销的是产品 C(已售出 150 件) 状态 C:最畅销的是产品 A(已售出 100 件)

目前我正在考虑在两列之间进行 JOIN，但转念一想，这需要一个类似“枢轴”的功能。

我知道如何获取每种产品的总销量

SELECT od.product_id, od.product_name, COUNT(od.product_id) AS 'total'
FROM `ps_order_detail` od
GROUP BY od.product_id
ORDER BY od.product_id DESC
LIMIT 100;

我还知道每个州的总销售量，但我不确定如何将两者结合起来来实现我的需要。我将不胜感激任何帮助。谢谢

Answer 1

您需要聚合查询的分组最大值。你可以这样做，例如像这样:

select state, product_name, quantity
from (
  select id_state, state, product_id, product_name, quantity,
    @grprow := (case when @grp = id_state then @grprow + 1 else 1 end) as grprow,
    @grp := id_state
  from  ( 
    SELECT a.id_state, max(s.name) as state, od.product_id, 
         max(od.product_name) as product_name,
         sum(od.product_quantity) as quantity
    FROM `ps_order_detail` od
    LEFT JOIN ps_orders AS o ON od.`id_order` = o.`id_order`
    LEFT JOIN `ps_address` AS a ON o.`id_address_delivery` = a.`id_address`
    LEFT JOIN `ps_state` AS s ON a.id_state = s.id_state
    group by a.id_state, od.product_id
  ) grped,
  (select @grprow := 0, @grp := null) init
  order by id_state, quantity desc, product_id
) grpmax
where grprow = 1
order by id_state;

它将每个州和产品的数量相加，然后对每个州的结果进行编号，然后取第一个。如果某个州的两种产品数量相同，则仅采用其中一种(product_id 较小的产品)。如果您想同时拥有两者，可以调整 @grprow 的编号条件。