gistfile1.txt

select sql_no_cache o.*

from (select * from orders where orders_status = 1) o

  where (
    select count(*) from orders_products op
    join products p on op.products_id = p.products_id
    join parts pa on p.products_id = pa.products_id
    where op.products_quantity > (
      p.products_physical_quantity - ifnull(
      (
        select sum(rs.quantity)
        from (select * from orders where orders_status in (1, 5)) o
        join reserved_stock rs on o.orders_id = rs.order_id
        where rs.part_id = pa.parts_id
      ), 0)
    )
    and op.orders_id = o.orders_id
  ) = 0

this is slower, but I think more readable and extensible. Hope this helps.

select sql_no_cache
  orders.*

FROM ( # the subset of new orders

  SELECT orders_id, date_purchased, orders_status FROM orders 
  WHERE orders_status = 1 # other constraints on orders can go here
  GROUP BY orders_id

) AS orders

INNER JOIN orders_products AS op ON orders.orders_id = op.orders_id

INNER JOIN ( # products with their reserve stock amounts

  SELECT DISTINCT parts.products_id, IFNULL(reserved_qty, 0) as reserved_qty
  FROM parts
  LEFT JOIN (

        # total reserved stock by part
        select part_id, sum(rs.quantity) as reserved_qty
        FROM reserved_stock AS rs
        WHERE rs.order_id in (

          #all orders contributing to reserved stock
          select distinct orders_id 
          from orders 
          where orders_status in (1, 5)

        ) 
        group by part_id

  ) as reserved ON parts.parts_id = reserved.part_id

) as parts ON op.products_id = parts.products_id

WHERE op.products_quantity>parts.reserved_qty
GROUP BY orders_id