SQL 如果下个月的列值不存在，则服务器查询以获取上个月的数据

Question

I have a table that contains a number of menu items in restaurants with respect to each month. and the table gets updated whenever there is a change in the number of items.

I want to change the table or make a new table to get the value of items for each month.

For example. If latest month is April

Restaurant	Number of items	Month
A	20	1/1/2021
A	15	1/21/2021
B	12	1/1/2021
C	30	2/1/2021
A	22	3/31/2021
B	15	4/1/2021

I want the new table to have the above rows plus the missing months with same data as last month

Restaurant	Number of items	Month
A	20	1/1/2021
A	15	1/21/2021
A	15	2/21/2021
A	22	3/1/2021
A	22	4/1/2021
B	12	1/1/2021
B	12	2/1/2021
B	12	3/1/2021
B	15	4/1/2021
C	30	2/1/2021
C	30	3/31/2021
C	30	4/1/2021

Thanks for the help guys appreciated

Answer 1

This answers the original version of the question.

Assuming that month is stored as a date with the first first day of the month, then a simple method uses recursive CTEs:

with cte as (
      select restaurant, num_items, month,
             dateadd(month, -1,
                     coalesce(lead(month) over (partition by restaurant order by month),
                              max(month) over ()
                             )
                    ) as end_month
      from t
      union all
      select restaurant, num_items, dateadd(month, 1, month), end_month
      from cte
      where month < end_month
     )
select *
from cte
order by restaurant, month;

Here is a db<>fiddle.