How the Database will Start

SQL> STARTUP NOMOUNT;

1.     Server process checks the parameter file at default location   "ORACLE_HOME/dbs". If spfile<sid>.ora is available Server process reads the spfile otherwise it will read init<sid>.ora file.

2.     If DB_NAME and CONTROL_FILES parameter is present in parameter file and the other optional parameters then Server process will allocate memory to the SGA.

3.     Server process creates entries in the trace file and alert log file.

4.     Instance creates and goes to nomount state and all the background processes will be started.

SQL> ALTER DATABASE MOUNT;

1.     It checks for the control files at location specified in parameter file. If control files are present it reads data files location and redo log files location and SCN number.

2.     Database goes to mount state.

SQL> ALTER DATABASE OPEN;

1.     It checks for the data files first and redo log files at location specified in control files. If present it checks the control file SCN number with data files and redo log files SCN number.

2.     If SCN number matches then database goes to open state.