Q139048: BUG: Update SQL Works Incorrectly with RAND() in WHERE Clause

Article: Q139048
Product(s): Microsoft FoxPro
Version(s): WINDOWS:3.0,3.0b,5.0,5.0a,6.0
Operating System(s): 
Keyword(s): kbprogramming kbvfp kbvfp300bBUG kbvfp500aBUG kbvfp500bugkbbuglist
Last Modified: 11-FEB-2000

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The information in this article applies to:

- Microsoft Visual FoxPro for Windows, versions 3.0, 3.0b, 5.0, 5.0a, 6.0 
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SYMPTOMS
========

Using the RAND() in an SQL UPDATE's WHERE clause can cause random results to
occur.

CAUSE
=====

The value of RAND() changes each time it is evaluated, unlike most FoxPro
functions. FoxPro's internal SQL parser evaluates it once to do the Rushmore
optimization and once more as each filter condition is tested for each record.
This causes unpredictable, random results.

RESOLUTION
==========

Instead of using the RAND() function in the WHERE clause, assign the value of
the RAND() function to a memory variable. Then use the variable in the WHERE
clause. For an example, please see the "Code to Correct Problem" section of this
article.

A new RAND() can be generated for every record tested in the filter by putting
RAND() in a User Defined Function (UDF) and calling the UDF in the SQL command
in place of RAND():

     SELECT * FROM <table> WHERE RAND() > .5

In this example the article's CAUSE section applies:

     SELECT * FROM <table> WHERE myudf() > .5
     PROCEDURE myudf()
     RETURN RAND()

STATUS
======

Microsoft has confirmed this to be a problem in the Microsoft products listed at
the beginning of this article. We are researching this problem and will post new
information here in the Microsoft Knowledge Base as it becomes available.

MORE INFORMATION
================

Code to Reproduce Problem
-------------------------

The following code illustrates the problem. If there wasn't a problem, the code
should make 1000 updates to the table. Instead, the update fails either 999 or
1000 times.

     *-- Code Begins Here
     CREATE TABLE testfile.dbf ;
          (mach_id i, mach_name c(10), db_key i, updcount i)
     INDEX ON db_key TAG db_key
     SET ORDER TO
     SET STATUS BAR ON

     FOR ii = 1 TO 1000
          INSERT INTO testfile.dbf VALUES (0, "", ii, 0)
     ENDFOR

     USE IN testfile
     USE testfile.dbf IN 0 SHARED
     SELECT testfile
     failcount = 0
     passcount = 0

     FOR ii = 1 to 1000
          *updval = INT(RAND() * 1000 + 1)
          UPDATE testfile ;
               SET mach_id = 1, ;
               mach_name = 'JOHNDOE', ;
               updcount = updcount+1 ;
               WHERE db_key = INT(RAND() * 1000 + 1)
          IF _TALLY = 0
               Failcount=Failcount + 1
          ELSE
               passcount=passcount + 1
          ENDIF
          SET MESSAGE TO ALLTRIM(STR(FAILCOUNT))+' Failed, ';
               +ALLTRIM(STR(PASSCOUNT))+' Passed'
     ENDFOR
     *-- Code Ends Here

Code to Correct Problem
-----------------------

To fix the problem, remove the '*' from the updval line and substitute
'updval'for 'INT(RAND() * 1000 + 1)' of the UPDATE's WHERE clause. The new line
would look like the following:

     WHERE db_key = updval

Additional query words: inconsistent kbvfp300 kbvfp500 kbvfp600

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Keywords          : kbprogramming kbvfp kbvfp300bBUG kbvfp500aBUG kbvfp500bug kbbuglist
Technology        : kbVFPsearch kbAudDeveloper kbVFP300 kbVFP300b kbVFP500 kbVFP600 kbVFP500a
Version           : WINDOWS:3.0,3.0b,5.0,5.0a,6.0
Issue type        : kbbug

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