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Microsoft
Access Resource Center
Using Nz() in expressions to generate correct results with Nulls
Provided by: Steve Clark, Senior Project Manager
Overview
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, VBA offers a way around this issue which can be used in code or Access 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).
Example
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.
Check out the VBA help file for the complete definition of the NZ function. You can add a second parameter for a non-zero value for nulls (for instance, you may want Nulls to be treated as 1).
More Information
For more information related to the Nz() function, you can review the following articles:
MSDN: Nz() Function
ACC2000: Adding the Values of Fields Returns a Blank Result If Some of the
Fields Contain Null Values
