PL/SQL Constants

Summary: in this tutorial, you will learn how to use the PL/SQL constants that hold values which do not change throughout the execution of the program.

Introduction to PL/SQL constants

Unlike a variable, a constant holds a value that does not change throughout the execution of the program.

Constants make your code more readable. Consider the following line of code that calculates the list price from price.

l_list_price :=  l_price + l_price * 0.1;

By looking at this, we don’t know what 0.1 means. It can be anything. Of course, you can use a comment to explain the meaning of 0.1:

-- price with value-added tax (VAT) 10%
l_list_price :=  l_price + l_price * 0.1;

However, it is even better if you use a constant like this:

l_list_price := l_price + l_price * co_vat;

In this code, co_vat is constant that stores the VAT tax 10%.

To declare a constant, you specify the name, CONSTANT keyword, data type, and the default value. The following illustrates the syntax of declaring a constant:

constant_name CONSTANT datatype [NOT NULL]  := expression

In this syntax:

constant_name

is the name of the constant that you are declaring.

datatype

specify the type of value that the constant will hold.

NOT NULL

optionally impose a NOT NULL constraint on the constant. This prevents the constant from storing NULL or an empty string.

expression

use an expression as the initial value for the constant. The type of the return value of the expression must be compatible with the data type of the constant.

PL/SQL constant examples

The following example declares two constants co_payment_term and co_payment_status:

DECLARE
    co_payment_term   CONSTANT NUMBER   := 45; -- days 
    co_payment_status CONSTANT BOOLEAN  := FALSE; 
BEGIN
    NULL;
END;

If you attempt to change the co_payment_term in the execution section, PL/SQL will issue an error, for example:

DECLARE
    co_payment_term   CONSTANT NUMBER   := 45; -- days 
    co_payment_status CONSTANT BOOLEAN  := FALSE; 
BEGIN
    co_payment_term := 30; -- error
END;

Here is the error message:

PLS-00363: expression 'CO_PAYMENT_TERM' cannot be used as an assignment target

The following illustrates how to declare a constant whose value is derived from an expression:

DECLARE 
    co_pi     CONSTANT REAL := 3.14159;
    co_radius CONSTANT REAL := 10;
    co_area   CONSTANT REAL := (co_pi * co_radius**2);
BEGIN
    DBMS_OUTPUT.PUT_LINE(co_area);
END;

In this example, the co_area constant receives the value from an expression involved two other constants.

In this tutorial, you have learned about the PL/SQL constants that hold values which remain unchanged throughout the execution of the program.