ORA-01157: cannot identify/lock data file 7 – see DBWR trace file

I dropped a tablespace, its corresponding data file was missing in the system. It showed that

SQL> drop tablespace SOE;
drop tablespace SOE
*
ERROR at line 1:
ORA-01157: cannot identify/lock data file 7 – see DBWR trace file
ORA-01110: data file 7: ‘/opt/oracle/product/11.2.0.3/db_1/dbs/soe.dbf’

SQL> !ls -l /opt/oracle/product/11.2.0.3/db_1/dbs/soe.dbf
/opt/oracle/product/11.2.0.3/db_1/dbs/soe.dbf: No such file or directory <– missing
Finally, I dropped it successfully by the following procedures

1. SQL> alter database datafile 7 offline drop;

Database altered.

2. SQL> select name from v$datafile;

NAME
——————————————————————————–
+DATA1/testdb/datafile/system.504.800535271
+DATA1/testdb/datafile/sysaux.505.800535271
+DATA1/testdb/datafile/undotbs1.506.800535273
+DATA1/testdb/datafile/users.507.800535273
+DATA1/testdb/datafile/example.519.800535661
+DATA1/testdb/datafile/undotbs2.520.800536373
/opt/oracle/product/11.2.0.3/db_1/dbs/soe.dbf

7 rows selected.

3. SQL> drop tablespace soe;

Tablespace dropped.

SQL>  select name from v$datafile;

NAME
——————————————————————————–
+DATA1/testdb/datafile/system.504.800535271
+DATA1/testdb/datafile/sysaux.505.800535271
+DATA1/testdb/datafile/undotbs1.506.800535273
+DATA1/testdb/datafile/users.507.800535273
+DATA1/testdb/datafile/example.519.800535661
+DATA1/testdb/datafile/undotbs2.520.800536373