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The control file is a file that the Oracle DBMS maintains to manage the state of the database, and it is probably the single most important file in the Oracle database Every database has one control file, but due to the file s importance, multiple identical copies (usually three) are maintained when the database writes to the control file, all copies of the file get written to The control file is critical to the functioning of the database, and recovery is difficult without access to an up-to-date control file Oracle creates the control file (and the copies) during the initial database creation process.

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All examples so far have used the if/then structure for comparing values. Usually, if the expression given to the test command or enclosed in the square brackets evaluates to true, then we perform some task. The syntax of this example is slightly different. In this code pattern, the test on the left of the && operator is performed as in previous comparison examples and based on the result (true or false); the code on the right is executed or skipped. Here, if the debug variable is set to 1, the left-hand expression evaluates to true and the echo command will be run. The line of code can also be read as test the condition AND execute the additional code if the test evaluates as true.

The control file contains the names and locations of the data files, redo log files, current log sequence numbers, backup set details, and the all-important system change number (SCN), which indicates the most recent version of committed changes in the database information that is not accessible by users even for reading purposes Only Oracle can write information to the control file, and the Oracle server process continually updates the control file during the operation of the database Control files are vital when the Oracle instance is operating When you turn the instance on, Oracle reads the control file for the location of the data and log files During the normal operation of the database, the control file is consulted periodically for necessary information regarding virtually every structure of the database The control file is also important in verifying the integrity of the database and when recovering the database.

The checkpoint process instructs the database writer to write data to the disk when some specific conditions are met, and the control file notes all checkpoint information from the online redo log files This information comes in handy during a recovery the checkpoint information in the control file enables Oracle to decide how far back it needs to go in recovering data from the online redo log files The checkpoint indicates the SCN up to which the data files are already written to the data files, so the recovery process will disregard all the information in the online redo log files before the checkpoint noted in the control file When you start an Oracle instance, the control file is consulted first, to identify all the data files and the redo log files that must be opened for database operations..

The main type of conditional control structure in PL/SQL is the IF statement, which enables conditional execution of statements. You can use the IF statement in three forms: IF-THEN, IF-THEN-ELSE, and IF-THEN-ELSEIF. Here s an example of a simple IF-THEN-ELSEIF statement: BEGIN . . . IF total_sales > 100000 THEN bonus := 5000; ELSEIF total_sales > 35000 THEN bonus := 500; ELSE bonus := 0; END IF; INSERT INTO new_payroll VALUES (emp_id, bonus . . .); END; /

PL/SQL loops provide a way to perform iterations of code for a specified number of times or until a certain condition is true or false. The following sections cover the basic types of looping constructs.

An alternative is available. If the logical AND (&&) is replaced with a logical OR (||), the additional code is executed only if the test returns false. This example can be read as test the condition OR execute the additional code (if the test evaluates as false).

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