Anonymous Block

An anonymous block is a PL/SQL block without a name for identification.It has to be brought to the server’s memory every time it is required .Anonymous Block can’t be accessed/referred neither by oracle nor by user for execution.It has to be compiled every time we need that block of statements

Stored Procedures

A procedure is a subprogram that performs a specific action . A stored procedure is a PL/SQL block that is stored in the database with a name. It is invoked using the name. Each procedure is meant for a specific purpose.
A stored procedure is stored in the database as an object. It is also called as database procedure as it is stored in the database.

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Packages

The packages contain stored subprograms, or standalone programs, which are
called the package’s subprograms. These subprograms can be called from
another stored program, triggers, precompiler programs, or any of the interactive Oracle
programs, such as SQL*Plus. Unlike the stored subprograms, the package itself cannot
be called or nested, and parameters cannot be passed to it.

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Triggers

A trigger is a PL/SQL block that is associated with a specific event, stored in a database,
and executed whenever that event occurs. Oracle supports four fundamental types
of triggers:
• Data manipulation language (DML) triggers
• Instead-of triggers
• Data definition language (DDL) triggers
• Database event triggers

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In either environment, the PL/SQL engine accepts as input any valid PL/SQL block or subprogram. Figure shows the PL/SQL engine processing an anonymous block. The PL/SQL engine executes procedural statements but sends SQL statements to the SQL engine in the Oracle database.

PL/SQL Engine

It is a separator,which splits the SQL and PL/SQL statements.PL/SQL block comprises of SQL &PL commands are submitted as one request.PL/SQL engine split the block and sends appropriated commands to the corresponding units.SQL evaluator will handle SQL statements(available on the server side).PL Evaulator to handle PL commands.Engine can reside either in client side or in the server side.

Where PL/SQL stands?

The client can interact with the system through front end application program.When there is a data oriented operation it invokes the PL/SQL program in single step.In PL/SQL end it approaches PL/SQL Engine which splits PL and SQL part separately and send the SQL request to the SQL.

Now SQL sends the data request through query to database.Data if available will be send back to SQL in turn to PL/SQL.Now PL/SQL will send the return values back to the client application

What Is PL/SQL?

PL/SQL stands for Procedural Language extensions to traditional SQL. PL/SQL is available as an enabling technology within other software products. PL/SQL does not exist as a standalone language. We can use PL/SQL in the Oracle database, in the Oracle Server, and in client side application development tools such as Oracle Forms. PL/SQL is closely integrated into the SQL language.PL/SQL adds programming constructs that are not native to SQL. PL/SQL allows us to combine SQL statements with standard procedural constructs.

PL/SQL Block Structure

PL/SQL is a block-structured language. That is, the basic units  that make up a PL/SQL program are logical blocks, which can contain any number of nested sub-blocks. Typically, each logical block corresponds to a problem or subproblem to be solved.

A block (or sub-block) lets you group logically related declarations and statements. That way, you can place declarations close to where they are used. The declarations are local to the block and cease to exist when the block completes.

a PL/SQL block has three parts: a declarative part, an executable part, and an exception-handling part. (In PL/SQL, a warning or error condition is called an exception.) Only the executable part is required.

The Declaration Section

The DECLARATION section is optional. It is used to list the variables used in the
block along with the types of data they support. Cursors
are also declared in this section. This is the place where all local variables used
in the program are defined and documented.

The Execution Section

This section is the only one required. The contents must be complete to allow the
block to compile. By complete, we mean that a complete set of instructions for the
PL/SQL engine must be between the BEGIN and END keywords. As you saw earlier
with an execution section of NULL, compiled code does not mean it must actually
perform an action.
The execution section supports all DML commands and SQL*Plus built-in functions.
It supports DDL commands using Native Dynamic SQL (NDS) and/or the DBMS_SQL
built-in package.

The Exception Section

The EXCEPTION section is optional and traps errors generated during program
execution. This section starts with the reserved keyword “EXCEPTION” and can trap for specific errors using functions provided in the
STANDARD or DBMS_STANDARD packages or using EXCEPTION_INIT pragma
statements .

The PL/SQL Block structure is given below

DECLARE
--Declarations

BEGIN

--Statements

EXCEPTION

--Exception Handlers

END

An example of PL/SQL program is given below

DECLARE

emp_nam varchar2(30);

BEGIN

SELECT employee_name into emp_nam

FROM EMPLOYEES

WHERE emp_sal >100000;

EXCEPTION

---optional

WHEN NO_DATA_FOUND THEN

null;

END;

Nesting Of Blocks

Nesting of Blocks means blocks within the other blocks.sub blocks are allowed to enhance the modularity.inner blocks follow the similar syntax.The variables declared in the inner block is valid till the end of that block.There is no limitation on the number of nested blocks.

Local subprograms can be defined in the declarative part of any block.But such local subprograms can be called only from the block in which they are defined.They are local to that block which they are declared