Find users with activities in all the last 6 months - mysql

Question

I'm looking for the best solution on retrieving the list of users ID with activities in all the last 6 months.
Table structure and data, simplified, is the following:
CREATE TABLE activities (
id int,
client_id int,
created_at timestamp
);
insert into activities values
(1, 1, '2019-06-01 00:00:00'),
(2, 2, '2019-06-01 00:00:00'),
(3, 1, '2019-07-01 00:00:00'),
(4, 1, '2019-08-01 00:00:00'),
(5, 1, '2019-09-01 00:00:00'),
(6, 1, '2019-10-01 00:00:00'),
(7, 1, '2019-11-01 00:00:00'),
(8, 2, '2019-11-01 00:00:00'),
(9, 3, '2019-11-01 00:00:00');
I need to retrieve the list of users that has at least one activity for each one of the last 6 months. In the previous example just client_id 1
I thought doing a join, but it seems too expensive. I won't give you any idea on possible solutions in order not to deviate and accept whatever you have in mind.
Please, consider that I have to manage a really big data source (more then 50 millions rows).
Any quick idea?

Answer 1

I make no claims for the supremacy of this solution, partly because I find such requests disingenuous, but it should work, at least...
CREATE TABLE activities (
id int,
client_id int,
created_at timestamp
);
insert into activities values
(1, 1, '2019-06-01 00:00:00'),
(2, 2, '2019-06-01 00:00:00'),
(3, 1, '2019-07-01 00:00:00'),
(4, 1, '2019-08-01 00:00:00'),
(5, 1, '2019-09-01 00:00:00'),
(6, 1, '2019-10-01 00:00:00'),
(7, 1, '2019-11-01 00:00:00'),
(8, 2, '2019-11-01 00:00:00'),
(9, 3, '2019-11-01 00:00:00');
SELECT a.client_id
FROM activities a
WHERE a.created_at >= LAST_DAY(CURDATE() - INTERVAL 7 MONTH)+INTERVAL 1 DAY
GROUP
BY a.client_id
HAVING COUNT(DISTINCT(DATE_FORMAT(a.created_at,'%Y-%m'))) >= 6;
+-----------+
| client_id |
+-----------+
| 1 |
+-----------+