Lead window function in mysql to find sales

Given this table. I would like to know for each day how many different customers made a sale on date t and and t+1.

-- create a table
CREATE TABLE sales_t(
  id INTEGER PRIMARY KEY,
  d_date date NOT NULL,
  sale INT NOT NULL,
  customer_n INT NOT NULL
);
-- insert some values
INSERT INTO sales_t VALUES (1, '2021-06-30', 12,  1);
INSERT INTO sales_t VALUES (2, '2021-06-30', 22,  5);
INSERT INTO sales_t VALUES (3, '2021-06-30', 111,  3);
INSERT INTO sales_t VALUES (4, '2021-07-01', 27, 1);
INSERT INTO sales_t VALUES (5, '2021-07-01', 90, 4);
INSERT INTO sales_t VALUES (6, '2021-07-01', 33, 3);
INSERT INTO sales_t VALUES (6, '2021-07-01', 332, 3);

The result for date 2021-06-30 is 2 because customer 1 and 3 made a sale in t and t+1.

Date          sale_t_and_t+1
.....................................
2021-06-30             2 
2021-07-01             0

Answer

Use LEAD() window function for each distinct combination of date and customer to create a flag which will be 1 if the customer is present in both days or 0 if not and aggregate:

SELECT d_date, COALESCE(SUM(flag), 0) `sale_t_and_t+1`
FROM (
  SELECT DISTINCT d_date, customer_n,
         LEAD(d_date) OVER (PARTITION BY customer_n ORDER BY d_date) = d_date + INTERVAL 1 DAY flag
  FROM sales_t
) t
GROUP BY d_date;

See the demo.