Advanced SQL task: for each client, find the payment ID, client ID, and the number of days from the first payment.

To solve the task, it is necessary to determine the date of the first payment for each client, and then for each payment, calculate the difference in days from this date. Example in SQL (assuming a table payments with columns payment_id, client_id, payment_date):

SELECT
  p.payment_id,
  p.client_id,
  DATEDIFF(p.payment_date, first_pay.first_payment_date) AS days_from_first_payment
FROM
  payments p
JOIN
  (SELECT client_id, MIN(payment_date) AS first_payment_date
   FROM payments
   GROUP BY client_id) first_pay
ON p.client_id = first_pay.client_id;

Here, the subquery finds the first payment of each client, and the main query calculates the number of days between the current payment and the first one.