MYSQL loop in function not working - mysql

Question

I have a mysql function which will change records accordingly . But loop only execute for one time and leaves loop with this condition. "IF v_finished =1 THEN
LEAVE get_stock;
END IF;"
However its is supposed to execute multiple time. Like in my test case 3 times
BEGIN
DECLARE P_stock int(11);
DECLARE P_product int(11);
DECLARE V_From_warehouse int(11);
DECLARE V_To_warehouse int(11);
DECLARE v_finished INTEGER DEFAULT 0;
DECLARE V_To_warehouse_stock int(11);
DECLARE V_From_warehouse_stock int(11);
declare cur1 cursor for
SELECT material_transfer_details.product_id , material_transfer_details.quantity FROM
material_transfers,
material_transfer_details
WHERE
material_transfers.id = material_transfer_details.mtm_id
AND
material_transfers.status = 'Y'
AND
material_transfers.id = V_MTM_id;
DECLARE CONTINUE HANDLER
FOR NOT FOUND SET v_finished = 1;
SELECT warehouse_from INTO V_From_warehouse FROM material_transfers WHERE id =V_MTM_id;
SELECT warehouse_to INTO V_To_warehouse FROM material_transfers WHERE id =V_MTM_id;
OPEN cur1;
get_stock: LOOP
IF v_finished =1 THEN
LEAVE get_stock;
END IF;
fetch cur1 into P_product , P_stock;
SELECT quantity INTO V_To_warehouse_stock from stocks where warehouse_id = V_To_warehouse and product_id = P_product;
SELECT quantity INTO V_From_warehouse_stock from stocks where warehouse_id = V_From_warehouse and product_id = P_product;
IF (V_To_warehouse_stock IS NOT NULL)
THEN
UPDATE
stocks SET quantity = quantity - P_stock
WHERE
warehouse_id = V_to_warehouse
AND
product_id = P_product;
ELSE
INSERT INTO stocks(product_id , warehouse_id , quantity ,status, created_datetime , updated_datetime) values
(P_product , V_to_warehouse , 0-P_stock , 'Y', sysdate() , sysdate());
END IF;
IF (V_From_warehouse_stock IS NOT NULL)
THEN
UPDATE
stocks SET quantity = quantity + P_stock
WHERE
warehouse_id = V_from_warehouse
AND
product_id = P_product;
ELSE
INSERT INTO stocks(product_id , warehouse_id , quantity ,status, created_datetime , updated_datetime) values
(P_product , V_from_warehouse , P_stock , 'Y', sysdate() , sysdate());
END IF;
SET P_stock = 0;
SET P_product = 0;
END LOOP get_stock;
CLOSE cur1;
UPDATE material_transfers SET Status = 'N' WHERE id= V_MTM_id;
UPDATE material_transfer_details SET Status = 'N' WHERE mtm_id = V_MTM_id;
return '00000';
END

Answer 1

Two things:
First, change your code.
get_stock: LOOP
SET v_finished = FALSE;
fetch cur1 into P_product , P_stock;
IF v_finished =1 THEN
LEAVE get_stock;
END IF;
Since you are doing other things that could trip the handler, reset v_finished, then fetch from the cursor, and only then test whether to leave the loop.
As written, if you hadn't tripped the handler prematurely, you would have been testing in entirely the wrong place and would have stayed in the loop too long.
Next... be sure you understand SELECT ... INTO. I don't think it does quite what you think it does.
A scalar subquery is a much safer solution:
SET V_To_warehouse_stock = (SELECT quantity from stocks where warehouse_id = V_To_warehouse and product_id = P_product);
If SELECT ... INTO returns no rows, the variable's value does not change. It retains its former value, if one exists, and that is rarely what you expect.
Spooky action at a distance like this is best avoided, and it's an easy trap to fall into, in a loop.
See examples of this effect at https://dba.stackexchange.com/a/35207/11651.