How to find the Grandparent of this data

hi
I have some thing like
id node parent
1 A null
2 B A
3 C A
4 D B
5 E D
6 F D
7 G E
8 H G
9 I H
Assuming this is a product catalog, each one is a subcatalog under each subcategory , like A is the root , B and C are A's subcategories ,
I need to find out , given a node like I , I have to find not only the immediate , but say like grandparent's grandparent ie nth grandparent , kind of recursively finding it given a leaf level