What is PL/SQL?
* PL/SQL stands for the Procedural Language extension of SQL.
* The PL/SQL is the combination of the SQL along with procedural features of the programming languages.
* PL/SQL was developed by the Oracle Corporation in early 90's to increase the capabilities of the SQL.
The SQL statements are very powerful and concise, but do not do more as a group than they do individually. Normally speaking, the SQL statements operate independently, and having the little effect on one another. This is of the limited use for the writing programs, where you must make a body of the code that is going to vary its behavior according to data and to the user or the other input. To develop applications with the SQL, you usually have to either interface it to a standard programming language such as the C, or enlarge it so that it becomes a useful programming language in itself. The Oracle supports the both approaches, but latter approach has many advantages that are relevant to Web, and is therefore approach that Oracle Web Server takes.
The PL/SQL, then, is an application-development language that is a superset of the SQL, supplementing it with the standard programming-language features that include following:
1) Flow-control statements and the loops
2) Variables, types and constants.
3) Block or modular structure
4) Customized the error handling.
5) Structured data.
Another feature of the PL/SQL is that it allows you to store the compiled code directly in database. This enables any number of the applications or the users to share same functions and the procedures. In fact, once a given block of the code is loaded into the memory, any number of the users can use same copy of it simultaneously, which is useful for Oracle Web Server. The PL/SQL also enables you to define the triggers, which are the subprograms that database executes automatically in response to the specified events.
Define the PL/SQL Engine:
Oracle uses the PL/SQL engine to processes PL/SQL statements. A PL/SQL code can be stored in client system or client-side or in the database/server-side.
Basic Structure and Syntax of PL/SQL
The PL/SQL, like many programming languages, the groups statements into the units called blocks. These can either be named, in which the case they are called subprograms, or unnamed, in which the case they are anonymous blocks. Subprograms can be either procedures or functions. The difference between these, as in the most languages, is that a function is used in an expression and it returns a assessment to that expression, while a procedure is invoked as a separate statement and passes the values to calling program only through the parameters.
A block has three parts:
DECLARE section: - This is where you define the local variables, types, exceptions, constants and nested subprograms. The PL/SQL has forward declaration, but you can use it only for the subprograms. Therefore, you must define all the variables, types and constant and before referencing them. For more information on the forward declarations, see "Declaring Subprograms" in PL/SQL User's Guide and the Reference.
EXECUTABLE section: - Executable section is the actual code that the block executes. This is the only part of block that must always be present.
EXCEPTION section: - The Exception section is a section for handling the runtime errors and the warnings.
Variables and Constants
PL/SQL lets you declare the constants and variables, then use them in the SQL and procedural statements anywhere an expression can be used. The forward references are not acceptable. So, you must declare a constant or the variable before referencing it in other statements, including other declarative statements.
Control structures are the most important PL/SQL extension to SQL. Not only does the PL/SQL let you manipulate the Oracle data, it lets you process data using the conditional, sequential flow-of-control and iterative statements such as IF-THEN-ELSE, CASE, FOR-LOOP, WHILE-LOOP, EXIT-WHEN, and GOTO. Collectively, these statements can handle any situation.
Records contain the uniquely named fields, which can have the diverse data types. Suppose you have diverse data about an employee such as salary, name and hire date. These items are dissimilar in the type but rationally related. A record containing a field for each item lets you treat data as a logical unit.
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