Q168207: PRB: Cannot Find NULL Records with Parameterized Query

Article: Q168207
Product(s): Microsoft Visual Basic for Windows
Version(s): WINDOWS:5.0,6.0
Operating System(s): 
Keyword(s): kbGrpDSVBDB
Last Modified: 11-JAN-2001

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The information in this article applies to:

- Microsoft Visual Basic Professional Edition for Windows, versions 5.0, 6.0 
- Microsoft Visual Basic Enterprise Edition for Windows, versions 5.0, 6.0 
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SYMPTOMS
========

Running a parameterized query against a Jet database where the parameter
variable is equal to NULL will not return any records. Running a query without
the parameter variable that specifically searches for NULL will succeed as
expected.

CAUSE
=====

A variable with a NULL value sent in as the parameter is not equal to what the
JET engine recognizes as NULL. The values of the NULLs do not match.

RESOLUTION
==========

Before calling the code that executes the query, use an IF statement that checks
the value of the parameter. If the value is NULL, run a different query that
specifically looks for a NULL value, otherwise run the original, parameterized
query.

Example:

     Dim p1 as long
     dim r1 as recordset
     dim SQLstring as string
     dim p1 as long

     if p1 = NULL then
         SQLstring = "SELECT Table1.Field1 FROM Table1  _
         WHERE (((Table1.Field1) Is Null));"
         'Looking specifically for a Null
     else
         SQLstring = "SELECT Table1.Field1 FROM Table1 _
         WHERE (((Table1.Field1)=[" & p1 & "]));"
         'Parameterized query
     end if

     set r1 = MyDatabase.openrecordset(SQLstring, dbOpenDynaset)

STATUS
======

This behavior is by design.

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

Steps to Reproduce Behavior
---------------------------

1. Create a database with one table. Create one field in that table. Put a NULL
  (empty string) in the first record of the table.

2. Create two querydefs:

  a. Query1 will search specifically for records that equal NULL.

  b. Query2 will be the same as Query1 except that the search criteria is
     parameterized.

3. Run Query1.

4. Run Query2, setting the parameter variable as NULL.

Results:

Query1 will have one record while Query2 will have no records.

Additional query words: kbVBp500 kbVBp600 kbdse kbDSupport kbVBp kbNoKeyWord

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Keywords          : kbGrpDSVBDB 
Technology        : kbVBSearch kbAudDeveloper kbZNotKeyword6 kbZNotKeyword2 kbVB500Search kbVB600Search kbVBA500 kbVBA600 kbVB500 kbVB600
Version           : WINDOWS:5.0,6.0
Issue type        : kbprb

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