single table parent child relationship query

I have written a query to get the items from the table which doesn’t have any child items. It’s working fine but is very slow. Any better/easier/optimized way to write the same thing?

select distinct id, (select count(i.item_id) from order_item as i where i.parent_item_id = o.item_id) as c
from order_item as o
where product_type = 'bundle'
having c = 0
order by id desc
limit 10;

Few of the fields are these to get the idea of a structure

Table: order_item
Columns:
item_id PK
order_id
parent_item_id
product_id
product_type


item_id  | order_id | parent_item_id | product_id | product_type
-----------------------------------------------------------------
   1     |    1     |     null       |     1      |   bundle
   2     |    1     |       1        |     2      |   simple
   3     |    1     |       1        |     3      |   simple
   4     |    1     |     null       |     4      |   bundle
   5     |    2     |     null       |     1      |   bundle
   6     |    2     |       5        |     2      |   simple
   7     |    2     |       5        |     3      |   simple 

Query should only return the 4rth item

Answer

Try below. Also consider creating indexes on PARENT_ITEM_ID and ITEM_ID

SELECT OI.*
  FROM ORDER_ITEM OI
       LEFT JOIN ORDER_ITEM OI2
          ON OI2.PARENT_ITEM_ID = OI.ITEM_ID
 WHERE OI.PRODUCT_TYPE = 'bundle' AND OI2.PARENT_ITEM_ID IS NULL