Using NULL as a Parameter

See Also

Passing a null value as a parameter affects the behavior of many commands and functions in Visual FoxPro. The following general rules apply to null values passed to commands and functions:

• Commands generate errors when passed a null value.
  
  
• Functions that accept .NULL. as a valid value propagate .NULL. to the result.
  
  
• Functions that accept parameters that could be numeric values generate an error if you supply .NULL. for those parameters.

  STORE .NULL. TO n
  USE Mytable ALIAS &n            && Expected a workarea 
                                && name or number
  ? SUBSTR("Hello, world",n,5)   && Expected a position
                                && in the string
  

• ISBLANK( ), ISDIGIT( ), ISLOWER( ), ISUPPER( ), ISALPHA( ), and EMPTY return false (.F.) when passed a null value. ISNULL( ) returns true (.T.) when passed a null value.
  
  
• The commands INSERT - SQL and SELECT - SQL process null values through the IS NULL and IS NOT NULL clauses and, in the case of INSERT, UPDATE, and REPLACE, place null values into records.
  
  
• SQL Aggregate functions ignore instead of propagate null values.
  
  
• Visual FoxPro aggregate functions only propagate .NULL. if all supplied values are null values; otherwise, any null value is ignored.

For more information on using null values in your application, see Chapter 7, Working with Tables, in the Programmer's Guide.