How to update and insert into db with #transactional on db query - mysql

Question

I have one table warehouse_item_mapping
having columns - warehouseId, itemId, stockQuantity
warehouse_id item_id stock_quantity
1 123 10
1 234 20
1 345 30
This table is being used as inventory management as maintaining current stock of items in warehouse.
Now if new stock has come with some items including new items with current stock quantity.
Like
1 123 50
1 234 50
1 678 50 (new item)
I have to update the stock quantity in table as sum of existing quantity and current stock quantity. And if mapping not found in table then insert that new mapping.
For this case table should be updated as
warehouse_id item_id stock_quantity
1 123 60
1 234 70
1 345 30
1 678 50
How to do this in single query.
Follow up question in this is
If someone used the current stock from table in between get and update query then how to maintain lock or transactional for whole flow.

Answer 1

INSERT ON DUPLICATE KEY UPDATE is what you need. In combination with pre selected stock_quantity. You need to make item_id PRIMARY KEY.
Now of course you will be using one SELECT and one INSERT INTO statement with PHP ( i guess) variables.
SELECT #stock_quantity:=stock_quantity FROM warehouse_item_mapping WHERE item_id=123;
INSERT INTO warehouse_item_mapping (warehouse_id, item_id, stock_quantity)
VALUES (1, 123, 50)
ON DUPLICATE KEY UPDATE warehouse_id=1, stock_quantity=#stock_quantity+50;
Working example: db-fiddle
RESULT:
Results
Query #1 Execution time: 0ms
warehouse_id item_id stock_quantity
1 123 60
1 234 70
1 345 30
1 678 50
SO lats say you are using PHP:
PHP:
'$item_id' // your new OR old inserting item id.
'$warehouse_id'
'$stock_quantity' // Your NEW stock number.
START TRANSACTION;
SELECT #stock_quantity:=stock_quantity
FROM warehouse_item_mapping WHERE item_id='$item_id';
INSERT INTO warehouse_item_mapping (warehouse_id, item_id, stock_quantity)
VALUES ('$warehouse_id', '$item_id', '$stock_quantity')
ON DUPLICATE KEY UPDATE warehouse_id='$warehouse_id', stock_quantity=#stock_quantity+'$stock_quantity';
COMMIT;
This will try to insert a value only if PRIMARY KEY item-id doesn't exists. If it does it will update other two fields.
EDIT:
I think you are confused what #transaction is, read about it here.
Above is example of one as you asked for it but this can also be done in one query as mentioned in comment. Example:
INSERT INTO warehouse_item_mapping (warehouse_id, item_id, stock_quantity)
VALUES (1, 123, 50) ON DUPLICATE KEY UPDATE
warehouse_id=1, stock_quantity=stock_quantity+50;
Working Fiddle.
PHP:
INSERT INTO warehouse_item_mapping (warehouse_id, item_id, stock_quantity)
VALUES ('$warehouse_id', '$item_id', '$stock_quantity') ON DUPLICATE KEY
UPDATE warehouse_id='$warehouse_id', stock_quantity=stock_quantity+'$stock_quantity';