Question

Table Name is Category.

CategoryId      ParentId        Name
1               NULL            StackOverFlow-1
2               1               StackOverFlow-2
3               1               StackOverFlow-3
4               2               StackOverFlow-4  
5               4               StackOverFlow-5

The parent of StackOverFlow-5 is StackOverFlow-4.

The parent of StackOverFlow-4 is StackOverFlow-2.

The parent of StackOverFlow-2 is StackOverFlow-1.

I want to make a function like below:

GetAllCategoryIdsUntilBaseParentByCategoryId(int Id)
{
    //..
}

I think it should be a recursive function. Isn’t it?

Pseude code:

int x -> Select ParentId From Category Where Id = 5
int y -> Select ParentId From Category Where Id = x
int z -> Select ParentId From Category Where Id = y

This model should go on where ParentId is null..

How can I do it?

Answer 1

With SQL Server 2008 you can do this in a elegant single CTE query:

Simplifying your schema as:

create table t (id int, parent_id int, c char(1));

insert into t values 
( 1, Null, 'A'),
  ( 2, 1, 'B'),
  ( 3, 1, 'C'),
     (4, 3, 'D' ),
  ( 5, 1, 'E' );

The query will be:

;with cte as (
  select *, id as node 
  from t 
  union all
  select t.*, cte.node
  from t inner join cte 
     on t.id = cte.parent_id
)
select * 
from cte
where node = 4;

Results:

| ID | PARENT_ID | C | NODE |
-----------------------------
|  1 |    (null) | A |    4 |
|  3 |         1 | C |    4 |
|  4 |         3 | D |    4 |

As you can see the recursion is in the query. This avoid to generate multiple queries and calls to database.