Simple Sql Statement

 Hi, I feel a difficulty to understand the following procedure, it is simple but i am not sure the logic, could anyone give me some explain / hints , then I know how to write the similiar procedure next time. thanks for your time.   I extract the code from the Classifieds Starter Kit

For the procedure of GetCategoryByParentId

WHERE    (ParentCategoryId = @ParentCategoryId)
OR (@ParentCategoryId = 0 AND ParentCategoryId IS NULL)


For the procedure of GetParentCategoryById

WHERE (SELECT Path
       FROM Category
       WHERE Id = @Id) LIKE Path + '%'



ALTER PROCEDURE GetCategoryByParentId
(
@ParentCategoryId int = 0
)
AS
SET NOCOUNT ON;
SELECT     [Id], [ParentCategoryId], [Name], NumProducts
FROM         Category
WHERE    (ParentCategoryId = @ParentCategoryId)
OR (@ParentCategoryId = 0 AND ParentCategoryId IS NULL)
ORDER BY Name


ALTER PROCEDURE GetParentCategoryById
@Id int
AS
SELECT Id, ParentCategoryId, Name, NumProducts
FROM Category
WHERE (SELECT Path
       FROM Category
       WHERE Id = @Id) LIKE Path + '%'
ORDER BY Path


Thanks a lot,
Trevor