SQL> alter database drop logfile group 2;
alter database drop logfile group 2
*
ERROR at line 1:
ORA-01624: log 2 needed for crash recovery of instance dbstat (thread 1)
ORA-00312: online log 2 thread 1: ‘/data1/dbstat/redo2.log’

SQL> alter system switch logfile;

System altered.

SQL> select * from v$Log;

    GROUP#    THREAD#  SEQUENCE#      BYTES    MEMBERS ARC STATUS           FIRST_CHANGE# FIRST_TIME
———- ———- ———- ———- ———- — —————- ————- ——————-
         1          1         12 2147483648          1 NO  ACTIVE                  203704 2009-09-17 19:04:23
         2          1         11   52428800          1 NO  ACTIVE                  203206 2009-09-17 18:56:08
         3          1         13 2147483648          1 NO  CURRENT                 203710 2009-09-17 19:04:38
SQL> alter system checkpoint;

System altered.

SQL> select * from v$Log;

    GROUP#    THREAD#  SEQUENCE#      BYTES    MEMBERS ARC STATUS           FIRST_CHANGE# FIRST_TIME
———- ———- ———- ———- ———- — —————- ————- ——————-
         1          1         12 2147483648          1 NO  INACTIVE                203704 2009-09-17 19:04:23
         2          1         11   52428800          1 NO  INACTIVE                203206 2009-09-17 18:56:08
         3          1         13 2147483648          1 NO  CURRENT                 203710 2009-09-17 19:04:38

SQL> alter database drop logfile group 2;

Database altered.

CHECKPOINT triggers Oracle for the checkpoint movement by the CKPT process, and write the contents of the dirt in the Data Buffer (already written in the record but not written in the DataFile) into the data file and release the space for the station. The DBW background process is completed, and the SCN of Controlfile and Datafile’s scn.
Generally executed because it is to delete a log, but if there is still content in the log to the data file, you need to check the data by hand, and then you can drop logfile group n.