CREATE FUNCTION [dbo].[GetChildCategories] (@parent_id int)  
RETURNS @work Table (num int IDENTITY(1,1),category_id int) AS  
BEGIN 
declare @childrenCount int,@currCategory_id int,@num int
set @num = 1
insert @work
select category_id
from Category_Classification
where parent_id = @parent_id
set @childrenCount = @@ROWCOUNT
while (@num <= @childrenCount)
    Begin
        select top 1 @currCategory_id = category_id
        From @work
        Where num = @num
        
        insert @work
        select category_id
        From Category_Classification
        where parent_id = @currCategory_id
        
        set @childrenCount = @childrenCount + @@ROWCOUNT
        set @num = @num + 1
    End
Return
END