Managing the NULL value

• In the data model editor:
  • the "Aceptar valores NULL" option in the data file description ("Details" tab). This option allows you to specify if the data file supports NULL values. In this case, you will have the ability to manage the NULL value for the different items of the data file.
  • the "Permitir valores NULL en los índices (compatible solo con la versión 28 y posteriores)" option in the data file description ("Details" tab). This option indicates that the key item can contain NULL values. Records whose indIices contain NULL values will be searchable.
    CAUTION: Once this feature is enabled, the generated index file is only compatible with versions 28 and later.
    Only applications compiled in version 28 and later will be able to access these data files.
    An error occurs if an application developed in version 27 or earlier attempts to access a file and this option is enabled: "Error, the file version is too recent".
  • a "Default to NULL" option available for each item of your files ("General" tab of the item description). This option allows you to define the null value as default value for the item.
  • a "NULL allowed" option for each item of your files ("General" tab of the item description). This option is used to allow (or not) the use of the NULL value for this item. By default, if NULL is supported by the data file, the "NULL Allowed" option is checked for all the items of the data file.
• In programming, several properties:
  

  Null	El Null Property se utiliza para: Asignar NULL al elemento para la actual Record. Averiguar si el artículo está asociado con el valor NULL para el Record actual. Define el valor de Default de un artículo cuando éste se describe mediante programación.
  NullableIndex	La propiedad NullIableIndex determina si un campo clave acepta valores "Null".
  NullAllowed	El NullAllowed Property se utiliza para gestionar el valor NULL en los elementos de los archivos de datos HFSQL.
  NullSupported	El NullSupported Property se usa para administrar el valor NULL en los archivos de datos HFSQL.

Limitations

• on the array items.
• on the composite keys.

How to save a NULL value in an item?

1. Check the "NULL supported" box when the data file is described in the data model editor ("Description of data file" in the context menu, "Info" tab).
   By default, "NULL Allowed" is checked for all file items.
2. Generate the analysis.
3. Check "NULL if empty" for the edit control linked to the item. If no value was entered by the user in this control, the NULL value will be automatically assigned to the item when using:
   • ScreenToFile.
   • PageToFile.

• The "NULL if empty" option for the edit controls can also be used to manage the optional parameters of a query with parameters. For more details, see Using a query with parameters.
• If the default value of the item is NULL (box checked in the editor), HReset resets the Null property to True.
• By default, the "Permitir valores NULL en los índices (compatible solo con la versión 28 y posteriores)" option is not checked. This means that the data file is compatible with older versions. NULL indexes are then considered as empty strings ("") in string items, or as the value 0 in numeric items.
  If the "Nueva pestaña" option is checked, the key item can contain NULL values. Records whose indIices contain NULL values will be searchable.

Customizing the display of the items whose value is NULL

Query: Selection condition

Example

• When managing the integrity, the NULL value is ignored.
• A linked record on 0 or empty string ("") is equivalent to a linked record on NULL.
• For backward compatibility, the transfer of Null by assignment is not allowed.
  • Don't:
    
    Fic1.Rub1 = Fic2.Rub2
  • Do:
    
    Fic1.Rub1..Null = Fic2.Rub2..Null

Novedad versión 2024

Nullable types: If "Permitir tipos que admiten valores NULL" is checked in the project description, you can use the following syntax:

Fic1.Rub1 = Fic2.Rub2

For more details, see Support NULL values: Allow nullable types.