Trigger for maintaining stock

Man, you can use stored procedure instead of trigger for this operation.

Inside the stored procedure, write Insert query for the Transaction Table & then for Stock table.

example looks like

CREATE PROCEDURE UspSALES
(...arguments...)
BEGIN

INSERT INTO SALES_TABLE(...fields...) VALUES(...values...)

--STOCK SHOULD BE DECREASED
UPDATE STOCK_TABLE SET (...fields...)=(...values...)

END

CREATE PROCEDURE UspPURCHASE
(...arguments...)
BEGIN

INSERT INTO PURCHASE_TABLE(...fields...) VALUES(...values...)

--STOCK SHOULD BE INCREASED
UPDATE STOCK_TABLE SET (...fields...)=(...values...)

END

please let me know if you have any doubt