如果您使用的是mysql 8.0，那么您可以使用 lag() 达到预期的产出。

select
    date,
    amount,
    coalesce((amount - last_val), amount) as diff
from
(
  select
      date,
      amount,
      lag(amount) over (order by date) as last_val
  from
  (
      select
          date,
          sum(amount) as amount
      from myTable
      group by
          date
  ) subq
) subo
order by
    date