MySQL Rolling Subtraction of a given qantity - mysql

Question

I have a table like this:
CREATE TABLE `PQ_batch` (
`id` int(6) unsigned NOT NULL AUTO_INCREMENT COMMENT 'Batch Id number',
`date` datetime DEFAULT NULL,
`qty` int(11) DEFAULT NULL COMMENT 'Number of units in a batch',
PRIMARY KEY (`bid`)
) ENGINE=InnoDB AUTO_INCREMENT=1000 DEFAULT CHARSET=utf8;
Id | date | qty
--------------------------------
1 2017-01-06 5
2 2017-01-02 5
3 2017-01-03 100
Given a qty value of: #qtyToTake:=100
*Select the rows that will be needed to fulfill the #qtyToTake and ONLY these rows, the quantity that is to be taken from each row, and the new quantity that remains for that row. The oldest batches should be used up first. *
It should look something like this:
Id | date | qty | newQty | qtyTakenPerRecord
-------------------------------------------------------
1 2017-01-02 5 0 5
2 2017-01-03 100 5 95
3 2017-01-01 5 5 0
newQty = (qty - #qtyToTake) where #qtyToTake = (#qtyToTake - the previous row's qty until #qtyToTake reaches 0)
#qtyToTake should be dynamically assigned to be the difference of the previous row's qty and its current value until it reaches 0.
Here's what I came up with:
SELECT p.bid, p.Orig as origQty, p.NewQty, (p.Orig - p.NewQty) AS NumToTake
FROM(
SELECT b.bid, (#runtot := b.bqty - #runtot) AS remain, ( #runtot := (b.bqty - #runtot) ) leftToGet, b.bqty AS Orig,
(SELECT
(sum(bqty) - #runtot) AS tot FROM PQ_batch
WHERE bid <= b.bid ) AS RunningTotal,
(SELECT
CASE
WHEN (sum(bqty) - #runtot) > 1 THEN (sum(bqty) - #runtot)
ELSE 0
END
FROM PQ_batch
WHERE bid <= b.bid ) AS NewQty
FROM PQ_batch b,(SELECT #runtot:= 100) c
ORDER BY bdate
) AS p

Answer 1

Using #McNets example here's what I came up with:
SELECT bid, bdate,bpid,bcost, bqty, newQty, qtyTakenPerRecord
FROM (
SELECT y.*,
IF(#qtyToTake > 0, 1, 0) AS used,
IF(#qtyToTake > bqty, 0, bqty - #qtyToTake) AS newQty,
IF(#qtyToTake > bqty, bqty, bqty - (bqty - #qtyToTake)) AS qtyTakenPerRecord,
#qtyToTake := #qtyToTake - bqty AS qtyToTake
FROM
(SELECT #qtyToTake := 100) x,
(SELECT * from PQ_batch WHERE bpid =1002 AND bqty > 0 ORDER BY bdate) y
) z
WHERE used = 1
ORDER BY bdate