with day_num as (
select '2021-01-01' fd_day, 1 num
union all select '2021-01-02', 2
union all select '2021-01-03', 3
union all select '2021-01-04', 4
union all select '2021-01-05', 5
)
select fd_day, num, sum(num) over(
order by fd_day
--前2条、本条、后0条记录的和
rows between 2 preceding and 0 following
) as sum_num, avg(num) over(
order by fd_day
--前2条、本条、后0条记录的均值,即3日均线
rows between 2 preceding and 0 following
) as avg_num
from day_num