Handling Nulls with GeneXus

The goal of this document is to explain how to work with -or avoid- null values in GeneXus-generated applications.

Attribute values

An attribute can have one of the following values:

1. An actual value.
2. Empty
   This is a special value, i.e. zero for numeric data, empty string for character data, etc.
3. Null, if it is allowed to
   Null means that the value is not set, unspecified, does not apply or unknown.

It is important to understand the difference between these last two special values: Null is not the same as Empty.

Working with null values

To be able to accept null values, an Attribute must be declared as nullable by setting the Nullable property - Attribute to Yes.

Nullable attributes can be set as null in different ways.

• Using SetNull method.
• Not referencing the attribute in the New commands or Transaction Structure  when Initialize not referenced attributes property is set to No.
• Assigning the Nullvalue() function to the attribute when the Generate null for nullvalue() property is set to Yes.
• Using the Transactions UI, by setting the Nulls in Form behavior property to Yes and let your application user to leave the attribute empty.
   

Constraints

• Variables cannot be null
• Nullity is not copied on assignments

If X is an attribute and its value is null, the statement "Y = X" does _not_ set the value of Y to null. It's set to empty. You can change this behavior by writing the following code:

If Y.IsNull()
  X.SetNull()
Else
  X = Y
Endif

• Nullity is not supported in parameter passing

Parameter passing protocol does not support nulls. The empty value is always received by the referenced object, even in the following situations:

• In a transaction there is a object.call(attribute) where the attribute is on the form and the control has Empty As Null = Yes.
• object.call(nullvalue(attribute)) and Generate null for nullvalue = Yes.
• object.call(attribute.setnull()).
  In this case a specification error is displayed.

Avoiding null values

The best way to avoid null values in an attribute is to set it's Nullable property - Attribute to No.

In backward compatibility situations or, if you cannot reorganize the database (to avoid nulls), there are several programming considerations you may take into account:

• Use the SetEmpty method instead of Nullvalue function

This minimizes the risk of setting a value to null. Nullvalue function behavior may be affected by the value of Generate null for nullvalue() property.

• Set the Initialize not referenced attributes property to Yes
• Set the Generate null for nullvalue() property to No

See also

Empty value for each DBMS/Data type