SQL UPDATE in a loop ( mysql , PHP ) - php

Question

I have two tables :
Order :
date | Product | Quantity
01/03| P1 | 2
01/03| P2 | 2
02/03| P1 | 3
02/03| P2 | 1
02/03| P3 | 5
Stock :
Purchase number | product | Quantity
01/03 | P1 | 4
01/03 | P2 | 1
02/03 | P2 | 2
02/03 | P3 | 5
02/03 | P1 | 1
02/03 | P1 | 1
The first table is my order (what I sold), with this table I want to update the second table (which is my stock) to know exactly what i have in stock.
But i want to do that following the date, I first update the Quantity.Stock 01/03 before 02/03 .
Right now I have a loop , the problem is I don't want to have any " -1 " and i don't want to update all the rows with the same data .
In this example I have 5 P1 in order so in stock the first line of P1 must be 0 , the next P1 line must be 0 too but the last line must stay at 1 .
Any ideas ? or direction ?
Bruno

Answer 1

I suppose date and purchase_number are some sort of datetime values.
Get orders, which you haven't used yet. I suppose you have some used/processed column or table, where you saved them as marked.
SELECT id, product, quantity
FROM orders
WHERE used = 0 AND quantity > 0
ORDER BY date ASC
Get all relevant purchases you can use in order you want to use them, so from the oldest.
SELECT id, product, quantity
FROM stock
WHERE quantity > 0
ORDER BY purchase_number ASC
You can then iteratively mark orders and update purchases. I assume you saved your results to $orders and $purchases respectively.
foreach ($orders as $order) {
$remaining = $order['quantity'];
foreach ($purchases as &$purchase)
if ($order['product'] !== $purchase['product']
|| $purchase['quantity'] === 0) {
continue;
}
$remaining = $purchase['quantity'] - $remaining;
$purchase['quantity'] = max($remaining, 0);
// update purchase, where
// :quantity is $purchase['quantity']
// :id is $purchase['id']
// UPDATE stock SET quantity = :quantity WHERE id = :id
if ($remaining >= 0) {
break;
}
$remaining = -$remaining;
}
unset($purchase);
if ($remaining > 0) {
// we have problem, we sold more, then we have in stock
}
// mark order as used, where :id is $order['id']
// UPDATE orders SET used = 1 WHERE id = :id
}
Example has time complexity O(M*N), where M is orders and N is purchases. You can change it to O(M+N), when you group data by products and do M SQL queries for purchases or some preprocessing. It has memory complexity O(M+N), which is a tradeoff for less queries.