Question

Consider the following rows in a database:

Id      |   Parent
__________________
1           null
2           1
3           2
4           3
5           null
6           5

Each Id that has a null Parent is the “Owner”/”Super Parent”.

What would be the best approach, performance wise, to collect the parents and their children ? Should I use LINQ or Stored Procedures ?

I want the end result to be IEnumerable<IEnumerable<int>>.

Answer 1

In SQL, you could query using a CTE. For example, to retrieve a list of nodes with their parent and the highest parent in their tree:

declare @t table (id int, parent int)
insert @t (id, parent) values (1, null), (2,1), (3,2), (4,3), (5,null), (6,5)

; with cte as (
    select  id, parent, id as head
    from    @t
    where   parent is null
    union all
    select  child.id, child.parent, parent.head
    from    @t child
    join    cte parent
    on      parent.id = child.parent
)
select  *
from    cte

This gives:

id  parent  head
1   NULL    1
2   1       1
3   2       1
4   3       1
5   NULL    5
6   5       5

Note that I changed your example data so row 2 is no longer a child of itself, but a child of row 1.