Question

I have a table which stores categories and sub categories (self join)

The table structure is like this:
-CategoryId
-CategoryName
-ParentCategoryId

How to find nth child/childs of a category in sql query

Answer 1

How about something like this

DECLARE @Categories TABLE(
        CategoryId INT, 
        CategoryName VARCHAR(20),
        ParentCategoryId INT
)

INSERT INTO @Categories SELECT 1, '1',NULL
INSERT INTO @Categories SELECT 2, '2',NULL

INSERT INTO @Categories SELECT 3, '1.3',1
INSERT INTO @Categories SELECT 4, '1.4',1

INSERT INTO @Categories SELECT 5, '1.3.5',3
INSERT INTO @Categories SELECT 6, '1.3.6',3
INSERT INTO @Categories SELECT 7, '1.3.6.7',6

INSERT INTO @Categories SELECT 8, '1.4.8',4

DECLARE @CatID INT,
        @NthLevel INT
SELECT  @CatID = 1,
        @NthLevel = 2

;WITH Vals AS (
        SELECT  *,
                1 AS CatLevel
        FROM    @Categories c 
        WHERE   CategoryId IS NULL
        UNION ALL
        SELECT  c.*,
                CatLevel + 1 AS CatLevel
        FROM    Vals v INNER JOIN
                @Categories c   ON  c.ParentCategoryId =  v.CategoryID
)
SELECT  *
FROM    Vals
WHERE   CatLevel = @NthLevel

This will recursively build the tree structure, and limit it on the tree level you are looking for.