上个月的SQL结转

Question

I have some data that I am trying to get some counts on. There are dates for when the record was entered and when it was closed, if it has been closed yet. I want to be able to get a count of how many records were still open from the previous month as of the first of the month. Here is an example. First table is the data, second table is the results I am looking for. In the second table, ignore the parenthesis, they are just the IDs of the records that make up that count.

Position    DateEntered   DateClosed
    1        12/15/2017   12/20/2017
    11       12/20/2017    1/7/2018
    2        1/23/2018     2/3/2018
    3        1/24/2018  
    4        2/15/2018  
    5        2/20/2018     5/16/2018
    6        3/3/2018      3/15/2018
    7        3/23/2018     4/12/2018
    8        4/11/2018     5/10/2018
    9        4/12/2018     4/25/2018
    10       5/4/2018   

Year    Month      Carried Over
2018    January        1 (11)
2018    February       2 (2,3)
2018    March          3 (3,4,5)
2018    April          4 (3,4,5,7)
2018    May            4 (3,4,5,8)
2018    June           3 (3,4,10)
2018    July           3 (3,4,10)
2018    August         3 (3,4,10)

Is this possible, and if so, how? Been racking my brain on this one for a few hours.

Answer 1

For each month, you want the number of rows that start before that month and end after. I'm thinking:

with dates as (
      select cast('2018-01-01' as date) as dte
      union all
      select dateadd(month, 1, dte)
      from dates
      where dte < '2018-08-01'
    )
select d.dte,
       (select count(*)
        from t
        where t.dateentered < d.dte and
              (t.dateclosed > d.dte or t.dateClosed is null)
       ) as carriedover
from dates d;

Note that this puts the date in a single column, rather than splitting the year and month into separate columns. That is easily arranged, but I prefer to keep date components together.