An invisible column is a user-specified column whose values are only visible when the column is explicitly specified by name. You can add an invisible column to a table without affecting existing applications, and make the column visible if necessary.

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In general, invisible columns help migrate and evolve online applications. A use case might be an application that queries a three-column table with a SELECT * statement. Adding a fourth column to the table would break the application, which expects three columns of data. Adding a fourth invisible column makes the application function normally. A developer can then alter the application to handle a fourth column, and make the column visible when the application goes live.

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The following example creates a table products with an invisible column count, and then makes the invisible column visible:

CopyCREATE TABLE products ( prod_id INT, count INT INVISIBLE );
ALTER TABLE products MODIFY ( count VISIBLE );

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See also

• Oracle Database Administrator’s Guide to learn how to manage invisible columns
• Oracle Database SQL Language Reference for more information about invisible columns

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A table definition includes a table name and set of columns.

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A column identifies an attribute of the entity described by the table. For example, the column employee_id in the employees table refers to the employee ID attribute of an employee entity.

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In general, you give each column a column name, a data type, and a width when you create a table. For example, the data type for employee_id is NUMBER(6), indicating that this column can only contain numeric data up to 6 digits in width. The width can be predetermined by the data type, as with DATE.

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Virtual Columns

A table can contain a virtual column, which unlike a nonvirtual column does not consume disk space.

The database derives the values in a virtual column on demand by computing a set of user-specified expressions or functions. For example, the virtual column income could be a function of the salary and commission_pct columns.

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Oracle utilities are available to help you maintain the data in your Oracle Database.

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SQL*Loader

SQL*Loader is used both by database administrators and by other users of Oracle Database. It loads data from standard operating system files (such as, files in text or C data format) into database tables.

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Export and Import Utilities

The Data Pump utility enables you to archive data and to move data between one Oracle Database and another. Also available are the original Import (IMP) and Export (EXP) utilities for importing and exporting data from and to earlier releases.

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The V$PWFILE_USERS view contains information about users that have been granted administrative privileges.

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To determine which users have been granted administrative privileges:

• Query the V$PWFILE_USERS view.

Removing a Database Password File

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You can remove a database password file if it is no longer needed.

If you determine that you no longer require a database password file to authenticate users, then to remove it:

• Delete the database password file, and optionally reset the REMOTE_LOGIN_PASSWORDFILE initialization parameter to none.

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After you remove this file, only those users who can be authenticated by the operating system can perform SYSDBA, SYSOPER, SYSBACKUP, SYSDG, or SYSKM database administration operations.

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Use the GRANT statement to grant administrative privileges. Use the REVOKE statement to revoke administrative privileges.

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To grant the SYSDBA, SYSOPER, SYSBACKUP, SYSDG, or SYSKM administrative privilege to a user:

• Run the GRANT statement.

o revoke the administrative privilege from a user:

• Run the REVOKE statement.

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The WITH ADMIN OPTION is ignored if it is specified in the GRANT statement that grants an administrative privilege, and the following rules apply:

• A user currently connected as SYSDBA can grant any administrative privilege to another user and revoke any administrative privilege from another user.
• A user currently connected as SYSOPER cannot grant any administrative privilege to another user and cannot revoke any administrative privilege from another user.
• A user currently connected as SYSBACKUP can grant or revoke another user’s SYSBACKUP administrative privilege.
• A user currently connected as SYSDG can grant or revoke another user’s SYSDG administrative privilege.
• A user currently connected as SYSKM can grant or revoke another user’s SYSKM administrative privilege.

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Administrative privileges cannot be granted to roles, because roles are available only after database startup. Do not confuse the database administrative privileges with operating system roles.

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If you change the REMOTE_LOGIN_PASSWORDFILE initialization parameter from none to exclusive or shared, then you must ensure that the passwords stored in the data dictionary and the passwords stored in the password file for the non-SYS administrative users, such as SYSDBA, SYSOPER, SYSBACKUP, SYSDG, and SYSKM users are the same.

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Adding Users to a Database Password File

When you grant SYSDBA, SYSOPER, SYSBACKUP, SYSDG, or SYSKM administrative privilege to a user, that user’s name and privilege information are added to the database password file.

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A user’s name remains in the password file only as long as that user has at least one of these privileges. If you revoke all of these privileges, then Oracle Database removes the user from the password file.CompTIA certification malaysia

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Creating a Password File and Adding New Users to It

Use the following procedure to create a password file and add new users to it:

1. Follow the instructions for creating a password file as explained in “Creating a Database Password File with ORAPWD”.
2. Set the REMOTE_LOGIN_PASSWORDFILE initialization parameter to exclusive. (This is the default.)Oracle Database issues an error if you attempt to grant these privileges and the initialization parameter REMOTE_LOGIN_PASSWORDFILE is not set correctly.
3. Connect with SYSDBA privileges as shown in the following example, and enter the SYS password when prompted: CONNECT SYS AS SYSDBA
4. Start up the instance and create the database if necessary, or mount and open an existing database.
5. Create users as necessary. Grant SYSDBA, SYSOPER, SYSBACKUP, SYSDG, or SYSKM administrative privilege to yourself and other users as appropriate. 

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You can create a database password file with ORAPWD.

To create a database password file:

• Run the ORAPWD command.

Sharing and Disabling the Database Password File

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You use the initialization parameter REMOTE_LOGIN_PASSWORDFILE to control whether a database password file is shared among multiple Oracle Database instances. You can also use this parameter to disable password file authentication.

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To share a password file or disable password file authentication:

• Set the REMOTE_LOGIN_PASSWORDFILE initialization parameter.

You can set the REMOTE_LOGIN_PASSWORDFILE initialization parameter to one of the following values:

• none: Setting this parameter to none causes Oracle Database to behave as if the password file does not exist. That is, no privileged connections are allowed over nonsecure connections.
• exclusive: (The default) An exclusive password file can be used with only one database. Only an exclusive file can be modified. Using an exclusive password file enables you to add, modify, and delete users. It also enables you to change the password for SYS, SYSBACKUP, SYSDG, or SYSKM with the ALTER USER command.When an exclusive password file is stored on an Oracle ASM disk group, it can be used by a single-instance database or multiple instances of an Oracle Real Application Clusters (Oracle RAC) database.When an exclusive password file is stored on an operating system, it can be used with only one instance of one database.
• shared: A shared password file can be used by multiple databases running on the same server, or multiple instances of an Oracle RAC database, even when it is stored on an operating system. A shared password file is read-only and cannot be modified. Therefore, you cannot add users to a shared password file. Any attempt to do so or to change the password of SYS or other users with the administrative privileges generates an error. All users needing administrative privileges must be added to the password file when REMOTE_LOGIN_PASSWORDFILE is set to exclusive. After all users are added, you can change REMOTE_LOGIN_PASSWORDFILE to shared, and then share the file.This option is useful if you are administering multiple databases with a single password file.You cannot specify shared for an Oracle ASM password file.

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If REMOTE_LOGIN_PASSWORDFILE is set to exclusive or shared and the password file is missing, this is equivalent to setting REMOTE_LOGIN_PASSWORDFILE to none.

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If this argument is set to y, then ORAPWD deletes the specified password file. When y is specified, FILE, ASM, or DBUNIQUENAME must be specified. When FILE is specified, the file must be located on an ASM disk group.

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If this argument is set to n, the default, then ORAPWD creates the password file.INPUT_FILE

This argument specifies the name of the input password file. ORAPWD migrates the entries in the input file to a new password file. This argument can convert a password file from one format to another, for example from 12 format to 12.2 format.

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This argument also can be used to reset the password for the SYS administrative user.

When the INPUT_FILE argument is specified, ORAPWD does not create any new entries. Therefore, ORAPWD ignores the following arguments:

• PASSWORD
• SYSBACKUP
• SYSDG
• SYSKM

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When an input file is specified and the new password file replaces the input file, FORCE must be set to y.

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If password, then you will be prompted to enter the password for the SYSBACKUP administrative user.

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If external('sysbackup-external-name'), then replace sysbackup-external-name with the external name for SSL or Kerberos authentication for the SYSDG administrative user.

If global(sysbackup-directory-DN), then specify the directory service name for the global SYSBACKUP user.

SYSDG

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If password, then you will be prompted to enter the password for the SYSDG administrative user.

If external('sysdg-external-name'), then replace sysdg-external-name with the external name for SSL or Kerberos authentication for the SYSDG administrative user.

If global(sysdg-directory-DN), then specify the directory service name for the global SYSDG user.

SYSKM

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If password, then you will be prompted to enter the password for the SYSKM administrative user.

If external('syskm-external-name'), then replace syskm-external-name with the external name for SSL or Kerberos authentication for the SYSKM administrative user.

If global(syskm-directory-DN), then specify the directory service name for the global SYSKM user.

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If this argument is set to 12.2, the default, then ORAPWD creates a database password file in 12.2 format. 12.2 format is required for the password file to support granting administrative privileges to external users and SSL and Kerberos authentication for administrative users. Password profiles assigned to the users are also enforced on the administrative users.

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If this argument is set to 12, then ORAPWD creates a database password file in Oracle Database 12c format. Oracle Database 12c format is required for the password file to support SYSBACKUP, SYSDG, and SYSKM administrative privileges.

If this argument is set to legacy, then ORAPWD creates a database password file that is in the format before Oracle Database 12c. The password file supports SYSDBA and SYSOPER administrative privileges, but it does not support SYSBACKUP, SYSDG, and SYSKM administrative privileges.

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SYS

If SYS=Y and INPUT_FILE is specified to migrate password file entries, then you will be prompted to enter the new password for the SYS administrative user.

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If password, then you will be prompted to enter the password for the SYS administrative user.

If external('sys-external-name'), then replace sys-external-name with the external name for SSL or Kerberos authentication for the SYS administrative user.

If global(sys-directory-DN), then specify the directory service name for the global SYS user.

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