A package specification that does not need a package body is called a bodiless package. Only subprogram and cursor declarations in a package specification require an underlying implementation in the package body. If a package specification declares only those package constructs that do not require implementation details (e.g., types, constants, variables, exceptions, and call specifications), the corresponding package body does not need to be created. However, the body can be created to initialize constructs declared in the package specification.

Retrieving data using the SQL SELECT statement and using single row functions to customize the output

• /*…*/ and — are used to put comments in an Oracle program.
• The CREATE TABLE does not end immediately with a semicolon, and a column name cannot start with a numeric value. However, a numeric value can be used in between a column name.
• Name of a table cannot start with @ special character.
• Knowing about the GROUP BY clause.

Using sub queries and set operators to solve queries, as well as DDL statements to create and manage tables

• + (unary), – (unary), *, /, ||, =, !=, <, >, <=, >=, IS NULL, LIKE, BETWEEN, IN, NOT, AND, OR is the correct order of operators precedence.
• The ROLLBACK command is used to end the current transaction, and discard any changes made since the beginning of the transaction.

Managing objects with data dictionary views, schema objects and data in different time zones

• The length of a column can be increased even if it contains data. However, the data type of the column cannot be modified if it contains any data.
• The parameters used to maximize the space in Flash Recovery Area.
• User error failure refers to a database failure that results due to certain transactions on a database by a user.
• Flashback Query is used to view and repair historical data, and to view the contents of a table at any specific point in time.
• UNDO_RETENTION is a parameter in the init.ora initialization parameters file that specifies the time period in seconds for which a system retains undo data for committed transactions.
• Once the Flashback Versions Query is run to diagnose the erroneous transaction, the Flashback Transaction Query helps in the repair process of the transaction.
• The correct order of steps that will enable the flashback database are as follows:
  1. Put the database in Archive log mode.
  2. Set up a flash recovery area.
  3. Set the period for the flashback retention target.
  4. Shut down and mount the database.
  5. Enable flashback logging.
  6. Open the database.
• The overall utilization of the flash recovery area is displayed in megabytes.
• SQL statement can be used to flashback a database.
• Take the following steps to enable the Flashback Database:
  1. Ensure that the database is in Archive Log mode.
  2. Setup a flash recovery area.
  3. Set the duration for the flashback retention target.
  4. Shutdown the database.
  5. Mount the database.
  6. Enable flashback logging.
  7. Open the database.
• The dropped table can be recovered by using Flashback Drop.
• The flashback database is enabled only in Archive log mode, as it recovers the database to an earlier specific point of time.
• Flashback Database is used to reverse the changes made to a table.
• The Perform Recovery: Review screen is used to display the RMAN script that performs the Flashback Database recovery.
• The portlist.ini file is used to get all the port numbers of the HTTP listeners.
• RVWR is a background process, which is a part of the Flashback Database Architecture.
• LGWR is a background process that is a part of Flashback Database Architecture.
• SELECT oldest_flashback_scn FROM V; is used to retrieve an approximate system change number (SCN) to which a database can be flashed back.

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The REMOTE_OS_AUTHENT parameter in the parameter file of the database specifies whether remote clients will be authenticated with the value of the OS_AUTHENT_PREFIX parameter or not. The type of the REMOTE_OS_AUTHENT parameter is Boolean, the default value is false, and the parameter class is static. The REMOTE_OS_AUTHENT parameter is case sensitive on some operating systems. The flow diagram below may help a user understand the role of the REMOTE_OS_AUTHENT parameter for a remote database:

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The DBMS_SESSION PL/SQL package is used to provide access to the ALTER SESSION and ALTER SYSTEM statements. This package is also used for gathering session related information. The DBMS_SESSION PL/SQL package uses the privileges of the user executing this package, instead of using privileges of the user SYS.

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The UTL_TCP package enables PL/SQL applications to communicate with external TCP/IP-based servers by using TCP/IP. Because many Internet application protocols are based on TCP/IP, the UTL_TCP package is useful to PL/SQL applications that use Internet protocols.

The OPEN_CONNECTION function of the UTL_TCP package opens a TCP/IP connection with the specified remote and local host and port details. The remote host is the host providing the service. The remote port is the port number on which the service is listening for connections. The local host and port numbers represent those of the host providing the service. The OPEN_CONNECTION function returns a connection of PL/SQL record type.

The CLOSE_CONNECTION function of the UTL_TCP package closes an open TCP/IP connection. It takes the connection details of a previously opened connection as parameter. The CLOSE_ALL_CONNECTIONS procedure of the UTL_TCP package closes all open connections.

The READ_BINARY()/TEXT()/LINE() function of the UTL_TCP package receives binary, text, or text line data from a service on an open connection.

The WRITE_BINARY()/TEXT()/LINE() function of the UTL_TCP package transmits binary, text, or text line message to a service on an open connection.

Exceptions are raised when the buffer size for the input is too small, when generic network error occurs, when no more data is available to read from the connection, or when bad arguments are passed in a function call.

In case of local dependencies, the objects are on the same node in the database. The Oracle server automatically manages all local dependencies by using the database’s internal “depends-on” table. When a referenced object is modified, the dependent objects are invalidated. The next time an invalidated object is called, the Oracle server automatically recompiles it. If the automatic recompilation of the object fails, it remains invalid and Oracle server returns a run-time error.

For example, if the structure of a table on which a view is based is modified, the view becomes invalid. When the DESCRIBE command is used to describe the view, an error message is returned stating that the object is invalid to describe. However, if the view is queried, the view is recompiled automatically and the result is displayed if the view gets recompiled successfully.

If the definition of a referenced table is altered, the dependent procedures and functions become invalid. The dependent procedures and functions need implicit or explicit recompilation in order to become valid.

The recompilation of a dependent procedure or function is successful in the following cases:

When a private table is referenced by a dependent procedure, and the private table is dropped, the status of the dependent procedure becomes invalid. When the procedure is recompiled, either explicitly or implicitly, and a public table having the same name and structure exists, the procedure recompiles successfully but becomes dependent on the public table. The recompilation is successful only if the public table contains the columns that the procedure requires; otherwise, the status of the procedure remains invalid.

When a PL/SQL variable is declared to hold column values, it must be ensured that the variable is of the correct datatype and precision. If not, a PL/SQL error occurs during execution. Rather than explicitly specifying the datatype and precision of a variable, the %TYPE attribute can be used to declare a variable according to another previously declared variable or database column.

The %TYPE attribute is generally used when the value stored in the variable will be derived from a table in the database. In order to use the %TYPE attribute in place of the datatype that is required in the variable declaration, the attribute is prefixed with the table name and column name. If referring to a previously declared variable, the attribute is prefixed with the name of the variable.

When a PL/SQL block consisting of such variables is compiled, the PL/SQL determines the datatype and size of the variables in order to ensure that such variables are always compatible with the column that is used to populate it. This is a definite advantage for writing and maintaining code because there is no need to be concerned about column datatype changes made at the database level.

A subprogram is a named PL/SQL block that can accept parameters and be invoked. There are two types of subprograms: procedures and functions. A procedure is generally used to perform an action, and a function is used to compute a value. Subprograms can be stored at the server or application level. By using Oracle Developer components (Forms, Reports, and Graphics), procedures and functions can be declared as part of an application (a form or report) and be called from other procedures, functions, and triggers within the same application whenever necessary.

A function is similar to a procedure, except that a function must return a value to the calling environment, whereas a procedure may or may not return values to its calling environment.

Following is the syntax for creating a procedure:

CREATE [OR REPLACE] PROCEDURE procedure_name
[(parameter [mode] datatype, parameter [mode] datatype, ……)]
IS | AS
PL/SQL block;

where,