Q135753: Implications of Using NULL in Data Validation Rules

Article: Q135753
Product(s): Microsoft FoxPro
Version(s): WINDOWS:3.0
Operating System(s): 
Keyword(s): 
Last Modified: 15-FEB-2000

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The information in this article applies to:

- Microsoft Visual FoxPro for Windows, version 3.0 
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SUMMARY
=======

Because Data Validations are expressions, NULL values inserted into fields and
passed to Data Validation expressions behave in a consistent manner with that of
other expressions. If the Data Validation for a field resolves to True (.T.) or
False (.F.) with a NULL value, that logical result is accepted. In many cases,
the expression will evaluate to NULL (For example, NULL > 1000). In this
case, because the expression returns NULL, not True or False, it is rejected.

MORE INFORMATION
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Step-by-Step Example
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1. Create a program called Myprog.prg

2. Type in the following code segment:

     CREATE DATABASE mydata1
     CREATE TABLE mytable (lastname c(20) NULL, firstname c(20) NULL, ;
        city c(20) NULL CHECK city>"a")
     INSERT INTO mytable (lastname,firstname,city) ;
        VALUES ("Ansarti","Jim","San Jose")
     INSERT INTO mytable (lastname,firstname,city) ;
        VALUES ("Hayden","Rance",.NULL.)
     INSERT INTO mytable (lastname,firstname,city) ;
        VALUES ("Putnam","Phil","New Orleans")

3. Save the program.

4. In the Command window, type:

     DO Myprog.prg

5. The program executes, and the INSERT command with city equal to .NULL.
  generates the error message, "Field City Validation rule is violated."

Additional query words: 3.00 VFoxWin

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Keywords          :  
Technology        : kbVFPsearch kbAudDeveloper kbVFP300
Version           : WINDOWS:3.0

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