recursive processing

Hi All,

I have two tables.  The first table (tbl_item) contains an item id and
it's type.

tbl_item
item_id | item_type
--------+----------
      A | DIR
      B | DIR
      C | ASY
      D | DIR
      E | DIR
      F | DIR
      G | ASY

The second table (tbl_assembly) contains the components of the
assemblies.  It is possible that an assembly is made up of any quantity
of DIR items or one or more assemblies

tbl_assembly
item_id | component_id | quantity
--------+--------------+---------
      C |            A | 2
      C |            B | 4
      G |            C | 3
      G |            E | 1
      G |            F | 8

I would like to perform some recursive processing to replace any
assembly used as a component with the appropriate number of components
so that all component_ids are of item_type = 'DIR'.

item_id | component_id | quantity
--------+--------------+---------
      C |            A |  2
      C |            B |  4
      G |            A |  6
      G |            B | 12
      G |            E |  1
      G |            F |  8

I want to perform this processing any time an item_id is INSERTed or
UPDATEDed into tbl_assembly (TRIGGER) and place this result back into
the assembly table.

Any assistance and URL's to documentation or examples is appreciated.

--
Kind Regards,
Keith