title: Find out if a product has been changed
body: <p>I would like to know if a customer has cancelled one product (finish_date) and started another (start_date) within one month</p>
  <pre><code>CREATE TABLE `subscription` (
    `customer_number` char(7) COLLATE utf8mb4_unicode_ci NOT NULL,
    `product_number` char(8) COLLATE utf8mb4_unicode_ci NOT NULL,
    `start_date` date DEFAULT NULL,
    `finish_date` date DEFAULT NULL
  ) ;
  
  INSERT INTO subscription (customer_number, product_number, start_date, finish_date) VALUES('1', '1', '2019-12-01', '2020-11-01');
  INSERT INTO subscription (customer_number, product_number, start_date, finish_date) VALUES('1', '2', '2020-12-13', NULL);
  INSERT INTO subscription (customer_number, product_number, start_date, finish_date) VALUES('2', '1', '2019-12-01', '2020-12-01');
  INSERT INTO subscription (customer_number, product_number, start_date, finish_date) VALUES('2', '2', '2020-12-13', NULL);
  </code></pre>
  <p><a href="http://sqlfiddle.com/#!9/ec202c" rel="nofollow noreferrer">http://sqlfiddle.com/#!9/ec202c</a></p>