Code:
SQL> startup
ORACLE instance started.
Total System Global Area 105978600 bytes
Fixed Size 453352 bytes
Variable Size 83886080 bytes
Database Buffers 20971520 bytes
Redo Buffers 667648 bytes
Database mounted.
ORA-01157: cannot identify/lock data file 2 - see DBWR trace file
ORA-01110: data file 2: 'D:\ORACLE\ORADATA\TEST\UNDOTBS01.DBF'
SQL> alter database datafile 'D:\ORACLE\ORADATA\TEST\UNDOTBS01.DBF' offline;
Database altered.
SQL> alter database open;
Database altered.
SQL> alter tablespace UNDOTBS1
add datafile 'D:\ORACLE\ORADATA\TEST\UNDOTBS02.DBF' size 200M;
Tablespace altered.
SQL> alter database datafile 'D:\ORACLE\ORADATA\TEST\UNDOTBS01.DBF'
offline drop;
Database altered.
SQL> shutdown immediate;
Database closed.
Database dismounted.
ORACLE instance shut down.
SQL> startup
ORACLE instance started.
Total System Global Area 105978600 bytes
Fixed Size 453352 bytes
Variable Size 83886080 bytes
Database Buffers 20971520 bytes
Redo Buffers 667648 bytes
Database mounted.
Database opened.