When i click button ? Inserted value in edittext should be total and Calculate to display in another edittext with the help SELECT QUERY?

All inserted in PaymentPaid(editText) value should be sum and display to totalamt(Edittext). '

Next.setOnClickListener(new View.OnClickListener() {

    public void onClick(View arg0) {

    try {

                String sql= Select-------?
        String amntpaid=PaymentPaid.getText().toString();       
        String totamnt=totalamt.getText().toString();
                }
                catch (Exception e) {
                e.printStackTrace();
                }
                }
            });`