MySQL Order by, then Update Inner Join - php

Question

I have a table categories which contains the columns ID & ParentID I would like to add a field called Level, which states which level in the category tree each category is.
I think i found my solution but it is in sql not mysql. So i have been converting it to the correct syntax. However, i think im missing a step. So, here is my code:
ALTER TABLE categories DROP Level;
ALTER TABLE categories ADD Level INT NULL;
UPDATE categories
SET Level = 0
WHERE ParentID IS NULL;
UPDATE categories AS A
INNER JOIN categories B ON A.ParentID = B.ID
SET A.Level = B.Level + 1
WHERE A.Level IS NULL AND
B.Level IS NOT NULL;
I think the problem may lie in the fact that in my DB The order of the categories do not come in any specific order, what i mean is as follows:
ID ParentID
2 NULL 0
4 55
7 2
.....more categories
55 2
So what i would like it do do is:
ID Parent Level
2 NULL 0
3 55 2
7 2 1
....
55 2 1
However, i think, but i might be wrong, is that i need to either order by ParentID first before i do the last operation, or my query is missformed.
I am not getting any errors however, but just not getting the results i am expecting this is what im getting;
ID Parent Level
2 NULL 0
3 55 NULL
7 2 1
....
55 2 1
Any ideas?

Answer 1

The question is how many levels do u have?
If they are 3 levels than u can do it like this
ALTER TABLE categories DROP Level;
ALTER TABLE categories ADD Level INT NULL;
UPDATE categories SET Level = 0 WHERE ParentID IS NULL;
UPDATE categories SET level = 1 where parentID = 2;
UPDATE categories SET level = 2 where parentID > 2;