category.ParentCategoryId = @parentCategoryId

 or

category.ParentCategoryId = null and this sql didn't return result, because expect category.ParentCategoryId is null.

Then I test another way to delive null value 

 if (parentCategoryId == null)
            {
                var query = from
                                    category in this.productsHelpDataContext.Categories
                            orderby
                                    category.Order ascending
                            where
                                category.ParentCategoryId == null

                            select category;

                return query.Select(category => category);
            }
            else
            {
                var query = from
                                    category in this.productsHelpDataContext.Categories
                            orderby
                                    category.Order ascending
                            where
                                category.ParentCategoryId == parentCategoryId
                            select category;

                return query.Select(category => category);
            }

In other word LINQ to SQL strict follow sql script convention.

DECLARE @TEST VARCHAR(30)
SET @TEST = '1,2,3,47, 89, 67'
DECLARE @Current INT


DECLARE @LISTINT TABLE (Number INT)

WHILE (LEN(@TEST) > 0)
BEGIN
    IF (Charindex(',', @TEST) > 0)
    BEGIN
        SET @Current = LEFT(@TEST, Charindex(',', @TEST) - 1)
        
        SET @TEST = RIGHT(@TEST, LEN(@TEST) - Charindex(',', @TEST))
        
    END
    ELSE
    BEGIN
        SET @Current = CAST(@TEST AS INT)
        SET @TEST = NULL
    END

    INSERT INTO @LISTINT VALUES(@Current)
    SET @Current = NULL
END

SELECT Number
FROM @LISTINT

whith is the best way to grow up with MS SQl Server?

I was doing paging in SQL Server 2000 using Temp Table or Derived Tables. I decided to checkout new function ROW_NUMBER() in SQL Server 2005. ROW_NUMBER() returns the sequential number of a row within a partition of a result set, starting at 1 for the first row in each partition. I have compared both the following query on SQL Server 2005.

SQL 2005 Paging Method

USE AdventureWorksGODECLARE  @StartRow INTDECLARE  @EndRow INTSET    @StartRow = 120SET @EndRow = 140SELECT   

 FirstName, LastName, EmailAddressFROM (SELECT    PC.FirstName, PC.LastName, PC.EmailAddress,ROW_NUMBER()

 OVER(Order BY PC.FirstName, PC.LastName,PC.ContactID) AS RowNumberFROM    Person.Contact PC) PersonContactWHERE  

  RowNumber > @StartRow AND RowNumber < @EndRowORDER BY FirstName, LastName, EmailAddressGO

SQL 2000 Paging Method

USE AdventureWorksGODECLARE  @StartRow INTDECLARE  @EndRow INTSET    @StartRow = 120SET @EndRow = 140CREATE TABLE

 #tables (RowNumber INT IDENTITY(1,1),FirstName VARCHAR(100), LastName VARCHAR(100), EmailAddress VARCHAR(100))

INSERT INTO #tables (FirstName, LastName, EmailAddress)SELECT    PC.FirstName, PC.LastName, PC.EmailAddressFROM   

 Person.Contact PCORDER BY FirstName, LastName, EmailAddressSELECT    FirstName, LastName, EmailAddressFROM   

 #tablesWHERE    RowNumber > @StartRow AND RowNumber < @EndRowDROP TABLE #tablesGO

if you wnat to make your life easiest just use ROW_NUMBE() instead using old temp table( or cursor related solutions).