Question

I have an SQL table like this

ID    | ParentID
1     | null
2     | null
3     | null
4     | 1
5     | 4
6     | 5
7     | 6
8     | 7
9     | 8

Now, as you can see, a child parent relationship is maintained. I want a query to select “all level” children of a given ID.

Suppose if I input ID = 1, the result should be

ID    | ParentID
1     | null
4     | 1
5     | 4
6     | 5
7     | 6
8     | 7
9     | 8

So all immediate children, as well as children of their children at any level, should come up.

Is it possible to do this in MS SQL server? I have MS-SQL server 2012.

Answer 1

Here is an example with an extra field Name, but with CTE the recursion is simple:

DECLARE @ID int

SET @ID = 1;

WITH CTE_Table_1
(
  ID,
  Name,
  ParentID,
  TreeLevel
)
AS(
 SELECT 
  ID,
  Name,
  ParentID,
  0 AS TreeLevel
 FROM Table_1
 WHERE ID = @ID

UNION ALL

 SELECT 
  T.ID,
  T.Name,
  T.ParentID,
  TreeLevel + 1
 FROM Table_1 T
 INNER JOIN CTE_Table_1 ON CTE_Table_1.ID = T.ParentID
)

SELECT * FROM CTE_Table_1