Using MySQL triggers to update customers balance

Question

I need some help in understanding triggers and how they work. I have 3 tables:

Customers
Id | Balance

Inv

Answer 1

You'll need two triggers - one for the invoice table:

delimiter //
CREATE TRIGGER add_invoice_to_balance AFTER INSERT ON invoices
FOR EACH
ROW
BEGIN
    UPDATE Customers SET balance = balance + NEW.Amount
      WHERE Customers.id = NEW.custid;
END;
//
delimiter;

And one for the payment table:

delimiter //
CREATE TRIGGER add_payment_to_balance AFTER INSERT ON payments
FOR EACH
ROW
BEGIN
    UPDATE Customers SET balance = balance - NEW.Amount
      WHERE Customers.id = NEW.custid;
END;
//
delimiter ;

fiddle here