Query specifications differ between nullable and non-nullable fields. For general information on nulls and nullable columns, see the chapters "Using the Tables Utility" and "Working with Data Types and Data Display Formats."

In a nullable column, blank fields are treated as nulls (no data). In a query operation, entering the equals sign (=) into a nullable numeric column without anything else tells QBF to look for and return only rows in which the specified field is null.

In a non-nullable column, blank fields are treated as if they contain a value of zero (0). Entering the equals sign (=) into a non-nullable column without entering anything else returns all rows containing a value of zero (0) in that field.