Null (Reserved word)

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Overview
Null and the queries
Ignoring the parameters: Null in HExecuteQuery
Parameters of a query coming from an edit control: Null if empty
Null and the variants
Null and the numeric values
Null and the WLanguage functions
Null and the dynamic objects

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Null (Reserved word)

Overview

The Null keyword can be used according to different methods in WLanguage:

To specify that a parameter must be ignored in a query. See "Null and the queries" in this page.
To specify that a variant variable contains no value. See "Null and the variants" in this page.
To compare a value to 0. See "Null and the numerics" in this page.
In some WLanguage functions, to specify that the parameter must be ignored. See "Null and the WLanguage functions" in this page.
To check whether a dynamic object is allocated or not. See "Null and the dynamic objects" in this page.

Null and the queries

Ignoring the parameters: Null in HExecuteQuery

When running a query with parameters with HExecuteQuery, some query parameters do not have to be specified. The query conditions that use parameters that are not specified or whose value is Null will be ignored.

Example: Let's consider the query named "Customer_LastName_FirstName" whose SQL code is as follows:

SELECT * FROM CUSTOMER WHERE LASTNAME = {Param1} AND FIRSTNAME = {Param2}

Case 1: The 2 parameters are specified
HExecuteQuery(Customer_LastName_FirstName, hQueryDefault, "Smith", "John")

will execute the query:
SELECT * FROM CUSTOMER WHERE LASTNAME = 'Smith' AND FIRSTNAME = 'John'

Case 2: Only the last name is specified
HExecuteQuery(Customer_LastName_FirstName, hQueryDefault, "Smith")

will execute the query
SELECT * FROM CUSTOMER WHERE LASTNAME= 'Smith'

Case 3: Only the first name is specified
// use the mandatory variant for the Null variable (not filled)
sLastName is Variant = Null
HExecuteQuery(Customer_LastName_FirstName, hQueryDefault, sLastName, "Vince")

Or
HExecuteQuery(Customer_LastName_FirstName, hQueryDefault, Null, "Vince")

will execute the query
SELECT * FROM CUSTOMER WHERE FIRSTNAME= 'John'

Parameters of a query coming from an edit control: Null if empty

In order for your query to be run even if no value was typed by the user, check "NULL if empty" for the edit controls ("Details" tab of control description).

When this option is checked, if the control is empty, the value of the parameter passed to the query will correspond to the NULL constant. Therefore, no error occurs when running the query: the conditions relative to this parameter will be ignored.

For example, the SQL code run is:

SELECT NameOfItems
FROM NameOfFiles
WHERE Item = {Param1}

The "Ex1" query is run in the "Btn_OK" button by HExecuteQuery. The WLanguage code used is as follows:

HExecuteQuery(Ex1, hQueryDefault, EDT_EditControl1)

In this code, EDT_EditControl1 corresponds to the control in which the user must enter the query parameter.

For this example, the table below describes the use of "NULL if empty":

Value entered by the user in EDT_EditControl1	"NULL if empty" option checked for EDT_EditControl1	SQL code run

No value	Checked option	SELECT NameOfItems FROM NameOfFiles
No value	Option unchecked	SELECT NameOfItems FROM NameOfFiles WHERE Item = ' '
A value is entered	Option checked or unchecked	SELECT NameOfItems FROM NameOfFiles WHERE Item = 'ValueEntered'

Null and the variants

To specify that a Variant variable contains no value, use the NULL constant.

Remarks:

For a variant type, NULL means "Not assigned"
For a numeric type, NULL means "equal to 0" (see below)
vVal is Variant
IF vVal = Null THEN ...
// The test returns True because the variant is not assigned
END

vVal = 0
IF vVal = Null THEN ...
// The test returns False because the variant is assigned
// with an integer whose value is 0
END

vVal = 5
IF vVal = Null THEN ...
// The test returns False because the variant is assigned
// with an integer whose value is 5
END

Null and the numeric values

Used with numeric values, Null enables you to compare a value to 0. The equality operators and the comparison operators can be used (= and <>).

Remarks:

For a variant type, NULL means "Not assigned" (see above)
For a numeric type, NULL means "equal to 0"
nVal is int
IF vVal = Null THEN ...
// The test returns True because nVal=0
END

nVal = 5
IF vVal = Null THEN ...
// The test returns False because nVal=5
END

Null and the WLanguage functions

Some WLanguage functions accept Null as parameter to specify that the parameter takes no value.

For example:


TreeAdd, TreeInsert	Null allows you to display no image for the different levels of added elements.
TreeListItem	Null is used to list the children from the root of the treeview.
TreeModify	Null allows you not to modify the image defined by TreeAdd
INIWrite	Null is used to remove a keyword or a section from the INI file

Null and the dynamic objects

For the dynamic objects (class, structure, array, automation object, ...), Null is used to find out whether the object is allocated or not.

For example:

// Declare a dynamic automation object
MyDynamicObject is dynamic Automation object
...

IF MyDynamicObject = Null THEN
// Create a dynamic automation object
MyDynamicObject = new object Automation dynamic MyServer
END