sudo nano /etc/mysql/my.cnf

ssl
ssl-cert=/etc/letsencrypt/live/pieterjan.pro/fullchain.pem
ssl-key=/etc/letsencrypt/live/pieterjan.pro/privkey-nopassword.key

pi@pieterjan:~ $ mysql -p -u root
Enter password:
mysql> show variables like '%ssl%';
+---------------+------------------------------------------------------------+
| Variable_name | Value                                                      |
+---------------+------------------------------------------------------------+
| have_openssl  | DISABLED                                                   |
| have_ssl      | DISABLED                                                   |
| ssl_ca        |                                                            |
| ssl_capath    |                                                            |
| ssl_cert      | /etc/letsencrypt/live/pieterjan.pro/fullchain.pem          |
| ssl_cipher    |                                                            |
| ssl_key       | /etc/letsencrypt/live/pieterjan.pro/privkey-nopassword.key |
+---------------+------------------------------------------------------------+