Mysql选择总和但最后三条记录

Question

我有一个包含客户日期和金额字段的表格 我想按日期汇总除每个客户的最后两个金额之外的按客户分组的金额 样本数据

 customer     date               amount
  a           2020-10-1             100
  a           2020-10-2             150
  a           2020-10-3             30
  a           2020-10-4             20
  b           2020-10-1             1
  b           2020-10-5             13
  b           2020-10-7             50
  b           2020-10-9             18

想要的结果

  Customer    Amount
   A          150
   B           14

类似

select Customer , 
SUM(amount- last 2 amount)
From TableA
Group By Customer

Answer 1

一个选项使用窗口函数，在 MySQL 8.0 中可用：

select customer, sum(amount) total_amount
from (
    select a.*, row_number() over(partition by customer order by date desc) rn
    from tablea a
) a
where rn > 2
group by customer

在早期版本中，另一种方法是使用相关子查询返回每个客户的第三个最晚日期以进行过滤：

select customer, sum(amount) total_amount
from tablea a
where date <= (select a1.date from tablea a1 where a1.customer = a.customer order by a1.date desc limit 2, 1)
group by customer