Update multiple rows with one value and divide it to the different row in single SQL query

Question

Basically I have two tables itemTable and stockTable, every time I add a stock, I will get the foreign key for itemId and add it in stockTable with quantity.

I want to update the stockquantity in stockTable every time the customer will buy a product.

PROBLEM: For example the customer will buy 7 shirt: SO FIRST WILL UPDATE ON THE STOCKID (3) QUANTITY OF 4 SECOND WILL UPDATE THE STOCKID (6) QUANTITY OF 3

SO THE TOTAL STOCKS OF SHIRT IS 3 AFTER UPDATING THE QUANTITY

THE STOCKID (3) WILL BECOME 0 THE STOCKID (6) WILL BECOME 3

HOW DO I QUERY THAT IN SQL. THANK YOU!

TOTAL STOCKS: (Base on the same itemName)
SHOES - 5
SHIRT - 10
BALL  - 15

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stockTable

stockId   itemId    stockQuantity  stockOut
1           35             5
3           89             4 
6           11             6
14          87             15

---

itemTable

itemId    itemName     itemSellPrice  supplierName
35           shoes          50          Jason
89           shirt          40          Jacob
11           shirt          65          Max
87           ball           150         Sam

Answer 1

First of all you need to identify the itemId(s) that must be updated, they are in the result of:

SELECT itemId FROM itemTable WHERE itemName = 'shirt'

Having said that the update query will look like this:

UPDATE stockTable SET ... WHERE itemId IN (SELECT itemId FROM itemTable WHERE itemName = 'shirt')

Obviously now we need to update the quantities, this must be done progressively until the order is complete starting from the lowerst itemId (for istance):

SELECT IF(stockQuantity>7,@remaining_qty:=stockQuantity-7,@remaining_qty:=@remaining_qty-stockQuantity) AS `remaining_qty` FROM stockTable WHERE itemId IN (SELECT MIN(itemId) FROM itemTable WHERE itemName = 'shirt';

UPDATE stockTable SET stockQuantity=IF(stockQuantity>7,stockQuantity-7,0) WHERE itemId IN (SELECT MIN(itemId) FROM itemTable WHERE itemName = 'shirt');

After that you can check the variable @remaining_qty, and reapeat the stuff until @remaining_qty>0.