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NULL values – PB Docs 126 – PowerBuilder Library

NULL values – PB Docs 126

NULL values

Description

Null means undefined or unknown.
It is not the same as an empty string or zero or a date of 0000-00-00.
For example, null is neither 0 nor not 0.

Typically, you work with null values only
with respect to database values.

Usage

Initial values for variables

Although PowerBuilder supports null values
for all variable datatypes, it does not initialize
variables to null. Instead, when a variable is
not set to a specific value when it is declared, PowerBuilder sets
it to the default initial value for the datatype—for example,
zero for a numeric value, false for boolean,
and the empty string (“”) for a string.

Null variables

A variable can become null if one of the
following occurs:

  • A null value
    is read into it from the database. If your database supports null, and
    a SQL INSERT or UPDATE statement
    sends a null to the database, it is written to
    the database as null and can be read into a variable
    by a SELECT or FETCH statement.

    note.png Null in a variable

    When a null value is read into a variable,
    the variable remains null unless it is changed
    in a script.

  • The SetNull function is used
    in a script to set the variable explicitly to null. For
    example:

Nulls in functions and expressions

Most functions that have a null value for any argument
return null. Any expression that has a variable
with a null value results in null.

A boolean expression that is null is considered
undefined and therefore false.

Testing for null

To test whether a variable or expression is null,
use the IsNull function. You cannot use
an equal sign (=) to test for null.

Valid This statement shows the
correct way to test for null:

Invalid This statement shows
the incorrect way to test for null:

Examples

Example 1

None of the following statements make the computer beep (the variable nbr is
set to null, so each statement evaluates to false):

Example 2

In this IF…THEN statement, the boolean
expression evaluates to false, so the ELSE is
executed:

Example 3

This example is a more useful application of a null boolean expression
than Example 2. It displays a message if no control has focus. When no
control has focus, GetFocus returns a null object
reference, the boolean expression evaluates to false,
and the ELSE is executed:


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