PRB: Can't Find NULL Records with Parameterized Query

• Microsoft Visual Basic Professional and Enterprise Editions for Windows, version 5.0

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.