FIX: Allow Nulls on Field Change Results of SELECT-SQL

• Microsoft Visual FoxPro for Windows, version 3.0

SYMPTOMS

The results of a three-table join are different if the key field is set to allow NULL values.

STATUS

Microsoft has confirmed this to be a problem in the Microsoft products listed at the beginning of this article. This problem was corrected in Microsoft Visual FoxPro 3.0b for Windows.

MORE INFORMATION

Steps to Reproduce Problem

1. Use the following commands to create three tables to use in the join.

   CREATE DATABASE grader

   CREATE TABLE students ;

         (student_id C(10), stu_lname C(30), stu_fname C(30))
   

   ALTER TABLE students ADD PRIMARY KEY student_id TAG student_id INSERT INTO students VALUES ;

         ('0000000001', 'Slade', 'Cameron')
   

   INSERT INTO students VALUES ;

         ('0000000002', 'Larson', 'Debby')
   

   INSERT INTO students VALUES ;

         ('0000000003', 'Kautzman', 'Pat')
   

   INSERT INTO students VALUES ;

         ('0000000004', 'Lund', 'Tres')
   

   CREATE TABLE classes ;

         (class_id C(6), room_num C(5), period C(2), ;
         subject C(30), instructor C(6))
   

   ALTER TABLE classes ADD PRIMARY KEY class_id TAG class_id INSERT INTO classes VALUES ;

         ('BUS125', '2/103', '1', 'Business Law', '')
   

   INSERT INTO classes VALUES ;

         ('HOM132', '1/101', '2', 'Home Economics', '')
   

   CREATE TABLE classmem ;

         (student_id C(10), class_id C(6), grade C(2))
   

   INDEX ON student_id TAG student_id INDEX ON class_id TAG class_id INSERT INTO classmem VALUES ('0000000001', 'BUS125', 'A') INSERT INTO classmem VALUES ('0000000002', 'BUS125', 'B') INSERT INTO classmem VALUES ('0000000003', 'BUS125', 'C') INSERT INTO classmem VALUES ('0000000001', 'HOM132', 'A') INSERT INTO classmem VALUES ('0000000002', 'HOM132', 'A') INSERT INTO classmem VALUES ('0000000004', 'HOM132', 'A')

2. Use the following SELECT command to join the three tables:

   SELECT stu_lname, stu_fname, subject, grade ;

         FROM students, classes, classmem ;
         WHERE students.student_id = classmem.student_id ;
         AND classmem.class_id = classes.class_id ;
         ORDER BY subject
   

   Notice that the result is six records.

3. Modify the structure of the Students table, and place a check in the NULL column of the student_id field so that it will allow NULL values.

   SELECT students MODIFY STRUCTURE

4. Rerun the SELECT command shown in step 2. In Visual FoxPro 3.0, the result is no records. In Visual FoxPro 3.0b, the result is six records as expected.