PL/SQL makes it easy to detect and handle errors.

When an error occurs, PL/SQL raises an exception. Normal execution stops and control transfers to the exception-handling part of the PL/SQL block. You do not have to check every operation to ensure that it succeeded, as in a C program.

For more information, see PL/SQL Error Handling.

The basic unit of a PL/SQL source program is the block, which groups related declarations and statements.

A PL/SQL block is defined by the keywords DECLARE, BEGIN, EXCEPTION, and END. These keywords divide the block into a declarative part, an executable part, and an exception-handling part. Only the executable part is required. A block can have a label.

Declarations are local to the block and cease to exist when the block completes execution, helping to avoid cluttered namespaces for variables and subprograms.

Blocks can be nested: Because a block is an executable statement, it can appear in another block wherever an executable statement is allowed.

You can submit a block to an interactive tool (such as SQL*Plus or Enterprise Manager) or embed it in an Oracle Precompiler or OCI program. The interactive tool or program runs the block one time. The block is not stored in the database, and for that reason, it is called an anonymous block (even if it has a label).

An anonymous block is compiled each time it is loaded into memory, and its compilation has three stages:

1. Syntax checking: PL/SQL syntax is checked, and a parse tree is generated.

2. Semantic checking: Type checking and further processing on the parse tree.

3. Code generation

For syntax details, see "Block".

<< label >> (optional)
DECLARE    -- Declarative part (optional)
  -- Declarations of local types, variables, & subprograms

BEGIN      -- Executable part (required)
  -- Statements (which can use items declared in declarative part)

[EXCEPTION -- Exception-handling part (optional)
  -- Exception handlers for exceptions (errors) raised in executable part]
END;

PL/SQL lets you declare variables and constants, and then use them wherever you can use an expression.

As the program runs, the values of variables can change, but the values of constants cannot.

For more information, see "Declarations" and "Assigning Values to Variables".

A PL/SQL subprogram is a named PL/SQL block that can be invoked repeatedly.

If the subprogram has parameters, their values can differ for each invocation. PL/SQL has two types of subprograms, procedures and functions. A function returns a result.

For more information about PL/SQL subprograms, see PL/SQL Subprograms.

PL/SQL also lets you invoke external programs written in other languages.

For more information, see "External Subprograms".

A package is a schema object that groups logically related PL/SQL types, variables, constants, subprograms, cursors, and exceptions.

A package is compiled and stored in the database, where many applications can share its contents. You can think of a package as an application.

You can write your own packages—for details, see PL/SQL Packages. You can also use the many product-specific packages that Oracle Database supplies. For information about these, see Oracle Database PL/SQL Packages and Types Reference.

A trigger is a named PL/SQL unit that is stored in the database and run in response to an event that occurs in the database.

You can specify the event, whether the trigger fires before or after the event, and whether the trigger runs for each event or for each row affected by the event. For example, you can create a trigger that runs every time an INSERT statement affects the EMPLOYEES table.

For more information about triggers, see PL/SQL Triggers.

Most PL/SQL input and output (I/O) is done with SQL statements that store data in database tables or query those tables. All other PL/SQL I/O is done with PL/SQL packages that Oracle Database supplies.

To display output passed to DBMS_OUTPUT, you need another program, such as SQL*Plus. To see DBMS_OUTPUT output with SQL*Plus, you must first issue the SQL*Plus command SET SERVEROUTPUT ON.

Some subprograms in the packages in Table 2-1 can both accept input and display output, but they cannot accept data directly from the keyboard. To accept data directly from the keyboard, use the SQL*Plus commands PROMPT and ACCEPT.

Data abstraction lets you work with the essential properties of data without being too involved with details.

You can design a data structure first, and then design algorithms that manipulate it.

Control statements are the most important PL/SQL extension to SQL.

PL/SQL has three categories of control statements:

• Conditional selection statements, which let you run different statements for different data values.

  For more information, see "Conditional Selection Statements".

• Loop statements, which let you repeat the same statements with a series of different data values.

  For more information, see "LOOP Statements".

• Sequential control statements, which allow you to go to a specified, labeled statement, or to do nothing.

  For more information, see "Sequential Control Statements".

Conditional compilation lets you customize the functionality in a PL/SQL application without removing source text.

For example, you can:

• Use new features with the latest database release, and disable them when running the application in an older database release.

• Activate debugging or tracing statements in the development environment, and hide them when running the application at a production site.

For more information, see "Conditional Compilation".

PL/SQL lets you issue a SQL query and process the rows of the result set one at a time.

You can use a basic loop, or you can control the process precisely by using individual statements to run the query, retrieve the results, and finish processing.

BEGIN
  FOR someone IN (
    SELECT * FROM employees
    WHERE employee_id < 120
    ORDER BY employee_id
  )
  LOOP
    DBMS_OUTPUT.PUT_LINE('First name = ' || someone.first_name ||
                         ', Last name = ' || someone.last_name);
  END LOOP;
END;
/

First name = Steven, Last name = King
First name = Neena, Last name = Yang
First name = Lex, Last name = Garcia
First name = Alexander, Last name = James
First name = Bruce, Last name = Miller
First name = David, Last name = Williams
First name = Valli, Last name = Jackson
First name = Diana, Last name = Nguyen
First name = Nancy, Last name = Gruenberg
First name = Daniel, Last name = Faviet
First name = John, Last name = Chen
First name = Ismael, Last name = Sciarra
First name = Jose Manuel, Last name = Urman
First name = Luis, Last name = Popp
First name = Den, Last name = Li
First name = Alexander, Last name = Khoo
First name = Shelli, Last name = Baida
First name = Sigal, Last name = Tobias
First name = Guy, Last name = Himuro
First name = Karen, Last name = Colmenares