Implications of Using NULL in Data Validation Rules

MORE INFORMATION

Step-by-Step Example

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."