Calculate Profit Based on First-In, First-Out Pricing

Question

Say I have purchase and sales data for some SKUs:

po_id | sku | purchase_date    | price | qty
----------------------------------------------
    1 | 123 | 2013-         


        

Answer 1

This is really horrible since it changes a MySQL variable in the queries, but it kind of works (and takes 3 statements):

select
  @income := sum(price*qty) as income,
  @num_bought := cast(sum(qty) as unsigned) as units
from sale
where sku = 123
;

select
  @expense := sum(expense) as expense,
  sum(units) as units
from (select
  price * least(@num_bought, qty) as expense,
  least(@num_bought, qty) as units,
  @num_bought := @num_bought - least(@num_bought, qty)
from purchase
where sku = 123 and @num_bought > 0
order by po_id
) as a
;

select round(@income - @expense, 2) as profit_margin;