MariaDB 10.2

By using db<>fiddle, you agree to license everything you submit by Creative Commons CC0.

CREATE TABLE t
    (`Id` int, `customer_id` int, `customer_name` varchar(4), `Date` varchar(10), `Books_bought` varchar(13))
;
    
INSERT INTO t
    (`Id`, `customer_id`, `customer_name`, `Date`, `Books_bought`)
VALUES
    (1, 1, 'Alex', '08.06.2017', 'HarryPotter1'),
    (2, 1, 'Alex', '08.06.2017', 'Harry Potter2'),
    (3, 1, 'Alex', '09.06.2017', 'PHP 7'),
    (4, 2, 'Sam', '10.06.2017', 'SQL')
;
SELECT
    customer_id, customer_name, `Date`, 
    GROUP_CONCAT(`Books_bought` ORDER BY `Books_bought` SEPARATOR ', ') AS books
FROM
    t
GROUP BY
    customer_id, `Date`
ORDER BY
    `Date`, customer_id ;


customer_id
customer_name
Date
books


1
Alex
08.06.2017
Harry Potter2, HarryPotter1

1
Alex
09.06.2017
PHP 7

2
Sam
10.06.2017
SQL

customer_id	customer_name	Date	books
1	Alex	08.06.2017	Harry Potter2, HarryPotter1
1	Alex	09.06.2017	PHP 7
2	Sam	10.06.2017	SQL