To run the database, you must specify the state in which you want your database to operate:

1- NOMOUNT.

2- MOUNT.

3- OPEN.

NOMOUNT:You can run the instance in this mode if you want to create a database or to re-create the Files Control file. When running the instance in this mode, the following steps occur:-

1- Read the variables file in the following order:-

* spfileSID.ora first 

* If it doesn't find it, it looks for .spfile.ora.

* If it doesn't find it, it looks for .initSID.ora.

2- Configure the SGA.

3- Run the Background Processes.

4- Open the alertSID.log file and the trace files.

MOUNT:

You can run the instance in this mode when performing some operations on the database such as changing the name of the data files or performing a complete database recovery or running the database in some modes such as Archive Log Mode, for example. When running the instance in this case, the following steps occur:

1- Access the Control files and open it after it has been specified by the variables file.

2- Read the Control files to specify the Data Files and know their status, as well as to specify the Redo Log Files.

OPEN:

This is the default mode when running the database so that the instance is open, the database is available and users can connect to the database and perform their operations.

While running in this mode the following steps occur:-

1- Open the Online Data Files.

2- Open the Online Redo Log Files.

This way the database is available to users, and if the database is open, for example, in the MOUNT OR NOMOUNT mode and you want to open it in the OPEN mode, you must use the ALTER command.