Reduce results based on previous record + LINQ + SQL

Question

I am trying to figure out how to do this with either LINQ or SQL.

I have a table called Activity, a process continously logs to it every day. This process checks for a certain condition (Found or Not). My table data looks like:

Date	Found
1/1/2021	0
1/2/2021	0
1/3/2021	1
1/4/2021	1
1/5/2021	0
1/6/2021	0
1/7/2021	0
1/8/2021	0
1/9/2021	0

I would like to reduce this result to show when a transition has occurred and the previous record (previous day):

Date	Found
1/2/2021	0
1/3/2021	1
1/5/2021	0

The only solution I can think of is to do a loop in C# and check for the changes and record them in a new collection. But I would really like to know if something like this is possible with SQL or LINQ.

Answer 1

Hmmm. . . you seem to want rows where the row, the previous row, or the next row has found = 1 . One method uses lag() and lead() :

select t.*
from (select t.*,
             lag(found) over (order by date) as prev_found,
             lead(found) over (order by date) as next_found
      from t
     ) t
where 1 in (prev_found, found, next_found);

You can also use a window frame:

select t.*
from (select t.*,
             sum(found) over (order by date rows between 1 preceding and 1 following) as num_found
      from t
     ) t
where num_found > 0;