If SystemD MariaDB/MySQL is failing to start, make sure your logs directory is set correctly. Sometimes this is caused by failing to correctly make sure the directory is moved.
You’ll see an error log in the journal as follows;
Nov 27 18:24:55 db-a mysqld: 2017-11-27 18:24:55 140273229838208 [Note] /usr/sbin/mysqld (mysqld 10.2.10-MariaDB-10.2.10+maria~jessie-log) starting as process 28677
Nov 27 18:24:55 db-a systemd: mariadb.service: main process exited, code=exited, status=1/FAILURE
Nov 27 18:24:55 db-a systemd: Failed to start MariaDB database server.
Nov 27 18:24:55 db-a systemd: Unit mariadb.service entered failed state.