row_number() in T-SQL - sql

Question

I have a table with these situation:
With my code
select
ID_A,
ID_B,
Position,
row_number() over (partition by ID_A, ID_B order by position) as row
from
TB_EXAMPLE
order by
ID_A, Position
I added a row_number_a column, but I want the row_number_b situation. Do you have any hints?

Answer 1

It's a gaps&islands problem, you need to assign a group number to each row before the Row_number. There are several ways, the following is based on the fact that the difference between two sequential number is the same as long as there's no gap in one of the sequences:
position rownum diff
1 1 0
2 1 1
3 1 2
4 1 3
5 2 3
3 3 3
...
22 1 21
23 2 21
24 2 22
Now all rows with the same ID_A and ID_B get the same difference if position is sequential and you can use this value in PARTITION BY:
with cte as
(
select
ID_A
,ID_B
,Position
,position -- if position is not sequential: ROW_NUMBER()over(partition by ID_A order by position)
- ROW_NUMBER()over(partition by ID_A,ID_B order by position) as grp
from TB_EXAMPLE
)
select
ID_A
,ID_B
,Position
,ROW_NUMBER()over(partition by ID_A, ID_B, grp order by position) as rn
from cte
order by ID_A,Position
Based on your comment that you actually want to delete rows, this can be simplified to a check if the previous row's values are the same as the current row's:
with cte as
(
select
ID_A
,ID_B
,Position
,case when LAG(ID_A)over(order by position) = ID_A
and LAG(ID_B)over(order by position) = ID_B
then 'delete'
else 'keep'
end as flag
from TB_EXAMPLE
)
select *
from cte
where flag = 'delete'
And as it looks like this is only based on changes in ID_B:
with cte as
(
select
ID_A
,ID_B
,Position
,case when LAG(ID_B)over(partition by ID_A order by position) = ID_B
then 'delete'
else 'keep'
end as flag
from TB_EXAMPLE
)
select *
from cte
where flag = 'delete'