Let's say you have a corrupted data file or lost a data file. Oracle can mount the database. But it will not open the database. What you can do is to set the bad data file as offline befor opening the database. The tutorial exercise shows you how to set two data files offline and open the database without them:

>sqlplus /nolog

SQL> connect SYSTEM/globalguideline AS SYSDBA

SQL> STARTUP MOUNT;

ORACLE instance started.

Total System Global Area 100663296 bytes

Fixed Size   1285956 bytes

Variable Size   58720444 bytes

Database Buffers   37748736 bytes

Redo Buffers   2908160 bytes

Database mounted.

SQL> ALTER DATABASE DATAFILE ' empmy_space.dbf'

2 OFFLINE DROP;

Database altered.

SQL> ALTER DATABASE DATAFILE ' empmy_space_2.dbf'

2 OFFLINE DROP;

Database altered.