Dynamic Average Calculations Against Partial Amounts of Distinct Groups - mysql

Question

We have a table representing purchases of a particular SKU. Let's say it has 4 columns, PurchaseNo, SKU, QTY and PRICE.
The QTY column represents the number of an SKU involved in each particular purchase. The PRICE column represents the dollar value paid for each unit on that particular purchase.
We need a query to determine the average PRICE of all currently “in stock” AG1TBA.
Let us assume that the QTY currently in stock is known to be 200. Let's also assume the top 3 rows of this table have the following QTY and PRICE information: 50, 22 | 120, 21 | 96, 25 |.
Since we only have 200 in stock, we somehow need the query to calculate the average from all 50 (at $22), all 120 (at $21) and 30 of the 96 (at $25). Obviously, the average price here is $21.85, but how could the query dynamically modify itself to limit the calculation of the average values to a specific amount of the the previous transactions (in this case, 200).
Here is an image of this hypothetical table:

Answer 1

Assuming the name of your table was endurion, you could solve it with following statements:
SET #stock := 200.0;
SET #amount := 0;
SELECT
SUM(e1.costs)/#stock as average_price_per_unit
FROM
(
SELECT
e.PurchaseNo,
e.SKU,
e.QTY,
#amount := #amount + e.QTY,
CASE
WHEN #amount <= #stock THEN e.QTY
ELSE e.QTY + #stock - #amount
END as part,
e.PRICE,
CASE
WHEN #amount <= #stock THEN e.QTY * e.PRICE
ELSE (e.QTY + #stock - #amount) * e.PRICE
END as costs
FROM (
SELECT
*
FROM
endurion e1 -- replace your table name here
WHERE
SKU = 'AG1TBA'
ORDER BY PurchaseNo DESC
) AS e
WHERE
#amount <= #stock
) as e1;
Explanation:
The innermost Subselect
SELECT
*
FROM
endurion e1
WHERE
SKU = 'AG1TBA'
ORDER BY
PurchaseNo DESC
ensures we match the latest purchases, because as you stated those will give us the current stock.
The outermost query will give us the average price per unit in stock. That should be obvious.
The outer subquery calculates the total of units by purchases. If this is greater than the stock, then only a part contributes to the stock. This part is calculated in the case statement and the second one calculates the total price for this units.
Hint
The complete query works for my installation of MySQL v5.6.16 but not in the v5.5.32 of the current sqlfiddle. I would consider to insert the result of
SELECT
e.PurchaseNo,
e.SKU,
e.QTY,
#amount := #amount + e.QTY,
CASE
WHEN #amount <= #stock THEN e.QTY
ELSE e.QTY + #stock - #amount
END AS part,
e.PRICE,
CASE
WHEN #amount <= #stock THEN e.QTY * e.PRICE
ELSE (e.QTY + #stock - #amount) * e.PRICE
END AS costs
FROM (
SELECT
*
FROM
endurion e1 -- replace your table name here
WHERE
SKU = 'AG1TBA'
ORDER BY PurchaseNo DESC
) AS e
WHERE
#amount <= #stock
into a temporary table and calculate the average by this temporary table.