Find parent-child relationship with database calls

I am having trouble setting up my database to allow to effectively delete cat/subcat relationships.

For my database I have the following setup:
the table name is galleryCategory
Fields:
categoryId
categoryName
categoryDescription
categoryParent
categoryGlobalParent

The theory behind this was that I would be able to flow from one to the next when creating the gallery for the front end.  Now the problem I am having deals with deleting a subcategory underneath another subcategory.  For example let's say that we have the following relationship:
1
-2
--3
---4
----8

I want to delete category 3 which has children 4 and 8 under it.  With my current setup I am racking my brain trying to find an effective and easy to implement solution to search the database for all the children in the database under category 3.  I tried to implement the categoryGlobalParent field to make it easier to narrow down but I still have yet to find a solution to this problem.

If I want to delete all of the categories under the main (1 in the example above) it is easy as I can use the global parent to run through all of them.  But the problem is when I am in the middle, how can I find all the id's  without knowing the number of categories below it to begin with?

What I tried doing, without success as I don't know how to do it, was to find all of the categories with the same global parent.  With that I knew how many total times I would have to do my loop.  The problem I am having is that I do not know how to place an unknown number of rows under one field into different variables.  Does anyone know of a simple way to do this?

Right now I have this:

categoryId:
[4]
[5]
[8]

I want to place each one of them either in a variable or an array so that I can access their values at a later time.

Thanks for the help.
-T