text: SELECT
      staff.staff_id,
      staff.first_name,
      staff.last_name,
      date_trunc('month', payment.payment_date) AS payment_month,
      sum(payment.amount) AS amount
  FROM
      staff,
      payment
  WHERE
      staff.staff_id = payment.staff_id
  GROUP BY
      staff.staff_id,
      staff.first_name,
      staff.last_name,
      date_trunc('month', payment.payment_date)
  LIMIT 10;
  
  