What are database startup options?

SQL> STARTUP
--> starts normally

SQL> STARTUP nomount
--> starts as nomount mode

SQL> STARTUP restrict
--> start as restrict mode

SQL> ALTER SYSTEM DISABLE RESTRICTED SESSION
--> unlock restrict mode

SQL> STARTUP force
--> tries to ignore startup issues

SQL> STARTUP open recover
--> starts as recover mode

SQL> ALTER DATABASE OPEN READ ONLY
--> starts as read only mode

SQL> ALTER DATABASE OPEN READ WRITE
--> starts unrestricted mode

In extra:
SQL> ALTER SYSTEM ENABLE RESTRICTED SESSION
--> enable restrict mode