Any arithmetic calculation in VBA with a Null value results in a null value. If one of the values is NULL, the result is Null (blank). According to VBA rules for calculating values, the result is null because a null value is an unknown.
Nulls are not the same value as zero (0). However, in many situations that's exactly what we want. We want to treat Nulls as zeros.
Thankfully, Microsoft Access offers a way around this issue in VBA module code or queries. The NullToZero function (NZ) takes a null value and converts it to zero. You should use this function anywhere a value could be null (like a field that is not required).
The following can be used to calculate the sum of two fields in the query:
Nz([Field1]) + Nz([Field2])
If [Field1] is null and [Field2] is 2, the result is 2. Without the Nz() function, the result would be null. So be safe! Wrap your potential null values with NZ.
There are two parameters. The second one is optional and lets you override the default value for nulls. For instance, you may want Nulls to be treated as 1, or for text fields, a zero-length string ("") rather than zero.
|Parameter||Required or Optional||Data Type||Description|
|Value||Required||Variant||Value to evaluate for Null|
|ValueIfNull||Optional||Variant||The custom return value if the value being evaluated is Null (overrides the default value)|
Check out the VBA help file for the complete definition of the NZ function.
On a related note, read our whitepaper Microsoft Access Query Sorting on Multiple Date or Numeric Fields with Blank (Null) Values where we discuss how the Immediate If command (IIF) is used to choose which field to sort and behaves differently from using the NZ function.
Note that the Nz Function is a Microsoft Access VBA function and does not exist in Visual Basic 6 (VB6) or other Office VBA environments.