PL/SQL Constants

As the name implies a constant is a value used in a PL/SQL Block that remains unchanged throughout the program. A constant is a user-defined literal value. You can declare a constant and use it instead of actual value.
For example: If you want to write a program which will increase the salary of the employees by 25%, you can declare a constant and use it throughout the program. Next time when you want to increase the salary again you can change the value of the constant which will be easier than changing the actual value throughout the program.
The General Syntax to declare a constant is:

constant_name CONSTANT datatype := VALUE;

constant_name is the name of the constant i.e. similar to a variable name.
The word CONSTANT is a reserved word and ensures that the value does not change.
VALUE - It is a value which must be assigned to a constant when it is declared. You cannot assign a value later.

For example, to declare salary_increase, you can write code as follows:

DECLARE

salary_increase CONSTANT number (3) := 10;

You must assign a value to a constant at the time you declare it. If you do not assign a value to a constant while declaring it and try to assign a value in the execution section, you will get a error. If you execute the below Pl/SQL block you will get error.

DECLARE

 salary_increase CONSTANT number(3);

BEGIN

 salary_increase := 100;

dbms_output.put_line (salary_increase);

END;

Introduction to PL/SQL

What is PL/SQL?

PL/SQL stands for Procedural Language extension of SQL.

PL/SQL is a combination of SQL along with the procedural features of programming languages. It was developed by Oracle Corporation in the early 90’s to enhance the capabilities of SQL.

The PL/SQL Engine:

Oracle uses a PL/SQL engine to processes the PL/SQL statements. A PL/SQL code can be stored in the client system (client-side) or in the database (server-side).

About This PL SQL Programming tutorial

This Oracle PL SQL tutorial teaches you the basics of programming in PL/SQL with appropriate examples. You can use this tutorial as your guide or reference while programming with PL SQL. I will be making this Oracle PL SQL programming tutorial as often as possible to share my knowledge in PL SQL and help you in learning PL SQL better.

Even though the programming concepts discussed in this tutorial is specific to Oracle PL SQL. The concepts like cursors, functions and stored procedures can be used in other database systems like Sybase , Microsoft SQL server etc, with some change in syntax. This tutorial will be growing regularly; let us know if any topic related to PL SQL needs to be added or you can also share your knowledge on PL SQL with us. Lets share our knowledge about PL SQL with others.
A Simple PL/SQL Block:

Each PL/SQL program consists of SQL and PL/SQL statements which from a PL/SQL block.

A PL/SQL Block consists of three sections:
•The Declaration section (optional).
•The Execution section (mandatory).
•The Exception (or Error) Handling section (optional).
Declaration Section:
The Declaration section of a PL/SQL Block starts with the reserved keyword DECLARE. This section is optional and is used to declare any placeholders like variables, constants, records and cursors, which are used to manipulate data in the execution section. Placeholders may be any of Variables, Constants and Records, which stores data temporarily. Cursors are also declared in this section.

Execution Section:
The Execution section of a PL/SQL Block starts with the reserved keyword BEGIN and ends with END. This is a mandatory section and is the section where the program logic is written to perform any task. The programmatic constructs like loops, conditional statement and SQL statements form the part of execution section.

Exception Section:
The Exception section of a PL/SQL Block starts with the reserved keyword EXCEPTION. This section is optional. Any errors in the program can be handled in this section, so that the PL/SQL Blocks terminates gracefully. If the PL/SQL Block contains exceptions that cannot be handled, the Block terminates abruptly with errors.

Every statement in the above three sections must end with a semicolon ; . PL/SQL blocks can be nested within other PL/SQL blocks. Comments can be used to document code.

This is how a sample PL/SQL Block looks.

DECLARE
Variable declaration
BEGIN
Program Execution
EXCEPTION
Exception handling
END;