Delete everything except certain value and the next two rows

Question

I need help to delete using SQL statement that I can delete everything except value contain 'Start' on Value1 field and the next two rows below that contain Keep* values. Bear in mind Keep* value can be other different values as well, I just use Keep as an example.

Thanks

**ID    DocumentName    Value1      Value2**
__  ____________    ______      _____
1   Doc1             AB        123rt
2   Doc1             CD        4543
3   Doc1             CE         fgf
4   Doc1             Csafsaf    fghfdg
5   Doc1             sdsaff     sdsa
6   Doc1             Start      From
7   Doc1            Keep2      Stay2
8   Doc1            Keep3      Stay3
9   Doc1            Stop        end
10  Doc1          hjkhjkiu     iuoiuio
11  Doc1          zxzfd         bxgx
12  Doc2           AB          123rt
13  Doc2          CD           4543
14  Doc2          CE           fgf
15  Doc2        Csafsaf     fghfdg
16  Doc2         sdsaff        sdsa
17  Doc2         Start         From
18  Doc2         Keep5        Stay5
19  Doc2         Keep6        Stay6
20  Doc2         Stop          end
21  Doc2        hjkhjkiu    iuoiuio
22  Doc2         zxzfd       bxgx
23  Doc3         dds          gdgd
24  Doc3         jhkjh       jhjkh
25  Doc3        jkkjh         fgf
26  Doc3      hnmnbjkhjkiu    mnbmn
27  Doc3        nmb           nbmn
28  Doc3       Start          From
29  Doc3       Keep7         Stay7
30  Doc3        Keep8        Stay8
31  Doc3        Stop          end

Basically I want to these values

**ID    DocumentName    Value1      Value2**
__  ____________    ______          _____
6   Doc1             Start      From
7   Doc1            Keep2      Stay2
8   Doc1            Keep3      Stay3
17  Doc2         Start         From
18  Doc2         Keep5        Stay5
19  Doc2         Keep6        Stay6
28  Doc3       Start          From
29  Doc3       Keep7         Stay7
30  Doc3        Keep8        Stay8

Answer 1

Use a CTE and lag() window function:

with cte as (
  select *,
    lag(Value1, 1) over (partition by DocumentName order by ID) prev1,
    lag(Value1, 2) over (partition by DocumentName order by ID) prev2
  from tablename  
)
delete from cte
where 'Start' not in (Value1, coalesce(prev1, ''), coalesce(prev2, ''))

See the demo.
Results:

> ID | DocumentName | Value1 | Value2
> -: | :----------- | :----- | :-----
>  6 | Doc1         | Start  | From  
>  7 | Doc1         | Keep2  | Stay2 
>  8 | Doc1         | Keep3  | Stay3 
> 17 | Doc2         | Start  | From  
> 18 | Doc2         | Keep5  | Stay5 
> 19 | Doc2         | Keep6  | Stay6 
> 28 | Doc3         | Start  | From  
> 29 | Doc3         | Keep7  | Stay7 
> 30 | Doc3         | Keep8  | Stay8