MySQL 8.0

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CREATE TABLE customer_bills (
   bill_date DATETIME,
   customer_id INTEGER
);
INSERT INTO customer_bills (Bill_Date, Customer_id)
VALUES ('2021-01-01', 1),
       ('2021-01-23', 2),
       ('2021-01-29', 3),
       ('2021-02-17', 1), 
       ('2021-02-19', 2), 
       ('2021-03-01', 3);

Records: 6  Duplicates: 0  Warnings: 0


WITH sub AS (
  SELECT DATE_ADD(MAKEDATE(YEAR(c.Bill_Date), 1), 
                  INTERVAL MONTH(c.Bill_Date)-1 MONTH) AS bill_month,
       COUNT(DISTINCT c.customer_id) AS unq_customers
  FROM customer_bills c
  GROUP BY DATE_ADD(MAKEDATE(YEAR(c.Bill_Date), 1), 
                    INTERVAL MONTH(c.Bill_Date)-1 MONTH)
)

SELECT bill_month,
       unq_customers,
       IFNULL(
         unq_customers /
         (SUM(unq_customers) OVER(ORDER BY bill_month) -
          unq_customers),
         1
       ) * 100 AS cumulative_retention_pct
FROM sub


bill_month
unq_customers
cumulative_retention_pct


2021-01-01
3
100.0000

2021-02-01
2
66.6667

2021-03-01
1
20.0000