Question

There is a table as follow:

Id   ||  ParentId ||  Other Columns ... 
=======================================
1    ||  1        || ...
2    ||  1        || ...
3    ||  1        || ...
1    ||  2        || ...
2    ||  2        || ...
3    ||  2        || ...
1    ||  3        || ...
2    ||  3        || ...

Column [Id] must have autoincremented value based on [ParentId] (own numeration).

What is the best way to achieve this goal?

Answer 1

try this

select ROW_NUMBER() over(partition by parentId order by <any other column>) ID,
ParentId,<other columns>  
from  yourtable

edit1:

If you want the id to be used in the where clause

with cte as(                                
select ROW_NUMBER() over(partition by parentId order by <any other column>) ID,
ParentId,<other columns>  
from  yourtable)
select * from cte where ID=(some value)