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.
Initial values for variables Although PocketBuilder 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, PocketBuilder 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.
The SetNull function is used in a script to set the variable explicitly to null. For example:
string city // city is an empty string.
SetNull(city) // city is set to NULL.
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:
IF IsNull(a) THEN ...
Invalid This statement shows the incorrect way to test for null:
IF a = NULL THEN ...
Example 1 None of the following statements make the computer beep (the variable nbr is set to null, so each statement evaluates to false):
int Nbr
// Set Nbr to NULL.
SetNull(Nbr)
IF Nbr = 1 THEN Beep(1)
IF Nbr <> 1 THEN Beep(1)
IF NOT (Nbr = 1) THEN Beep(1)
Example 2 In this IF...THEN statement, the boolean expression evaluates to false, so the ELSE is executed:
int a
SetNull(a)
IF a = 1 THEN
MessageBox("Value", "a = 1")
ELSE
MessageBox("Value", "a = NULL")
END IF
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:
IF GetFocus( ) THEN
. . . // Some processing
ELSE
MessageBox("Important", "Specify an option!")
END IF
