Null (Reserved word)

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Help / WLanguage / WLanguage syntax / Reserved keywords

Overview
Null and the queries
Ignoring the parameters: Null in HExecuteQuery
Query parameters 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 the query "Customers_first_name" have the following SQL code:

SELECT * FROM CLIENT WHERE NOM = {Param1} AND PRENOM = {Param2}

Case 1: The 2 parameters are given
```
HExecuteQuery(Clients_nom_prénom, hQueryDefault, "Dupond", "Jean")
```
will run the query:
SELECT * FROM CLIENT WHERE NOM = 'Dupond' AND PRENOM = 'Jean'

Case 2: Only the name is given
```
HExecuteQuery(Clients_nom_prénom, hQueryDefault, "Dupond")
```
will execute the query
SELECT * FROM CLIENT WHERE NOM = 'Dupond'

Case 3: Only the first name is given

// utilisation du variant obligatoire pour variable de type Null (non renseignée)
sNom is Variant = Null 
HExecuteQuery(Clients_nom_prénom, hQueryDefault, sNom, "Jean")

HExecuteQuery(Clients_nom_prénom, hQueryDefault, Null, "Jean")

will execute the query

SELECT * FROM CLIENT WHERE PRENOM = 'Jean'

Query parameters 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. No error will occur when running the query: the conditions that depend on this parameter will be ignored.

For example, the SQL code run is:

SELECT NomDesRubriques
FROM NomDesFichiers
WHERE Rubrique = {Param1}

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

HExecuteQuery(Ex1, hQueryDefault, SAI_ChampSaisie1)

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 ... 
	// Le test renvoie Vrai car le variant n'est pas affecté
END
 
vVal = 0
IF vVal = Null THEN ... 
	// Le test renvoie Faux car le variant est affecté 
	// avec un entier de valeur 0
END
 
vVal = 5
IF vVal = Null THEN ... 
	// Le test renvoie Faux car le variant est affecté
	// avec un entier de valeur 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 nVal = Null THEN ... 
	// Le test renvoie Vrai car nVal=0
END
 
nVal = 5
IF nVal = Null THEN ... 
	// Le test renvoie Faux car 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:

// Déclaration d'un l'objet automation dynamique
MonObjetDynamique is object Automation dynamic
...

IF MonObjetDynamique = Null THEN
	// Création d'un l'objet automation dynamique
	MonObjetDynamique = new object Automation dynamic MonServeur
END