PRB: C Datatype's Pseudo Null Values Are Not Inserted into SQL Database Correctly

• Microsoft Visual C++, 32-bit Editions, versions 5.0, 6.0

SYMPTOMS

When trying to insert or update records to a Microsoft SQL Server table, where the table contains fields of type Int, TinyInt, SmallInt, Real, or Float, some values known as Pseudo Nulls, are not inserted into the table. The Pseudo Null values are detailed in the table below.

CAUSE

If the CRecordset class is derived from a table in the Microsoft SQL Database, the MFC Wizard maps SQL Native datatypes to native Visual C Datatypes and calls the appropriate RFX functions in DoFieldExchange(). Some of the Visual C datatypes have values which are defined as Pseudo Nulls in Afxdb_.h. If an application tries to insert one of these Pseudo Null values into a table using CRecordset, they are regarded as NULL and nothing is inserted into the database. The following table shows the datatypes and their corresponding values which are regarded as Pseudo Null:

Table 1

RESOLUTION

Call SetFieldDirty() and SetFieldNull() before calling Update() on a Recordset object. A code example is given below.

STATUS

This behavior is by design.

MORE INFORMATION

Steps to Reproduce the Problem

1. Run this script on the pubs database in SQL Server using the Microsoft Query Tool. (Copy and paste might generate errors due to XML formatting).
   

   
   /****** Object:  Table dbo.INTTEST_TABLE ******/ 
   if exists (select * from sysobjects where id = object_id('dbo.INTTEST_TABLE') and sysstat & 0xf = 3)
   	drop table dbo.INTTEST_TABLE
   GO
   
   /****** Object:  Table dbo.INTTEST_TABLE ******/ 
   CREATE TABLE dbo.INTTEST_TABLE(
   id tinyint IDENTITY (1, 1) NOT NULL ,
   f1_tinyint tinyint NULL ,
   f1_smallint smallint NULL ,
   f1_int int NULL ,
   f1_real real NULL ,
   f1_float float NULL,
   CONSTRAINT PK_INTTEST_TABLE PRIMARY KEY  NONCLUSTERED(id) 
   )
   
   GO 




2. Create an ODBC DSN to SQL Server. Set pubs as the default database.



3. Create an MFC dialog based application.



4. Using the Class Wizard, create a CRecordset derived class (CTestSet) which uses the DSN you created in the step above and use the INTTEST_TABLE as our default table. (This is the table you created with SQL script).



5. Add the following to StdAfx.h.

   
   #include <afxdb.h> 
   #include "TestSet.h" 




6. Add a command button to the dialog resource. And add the following code to the command button handler:
   

   
           CTestSet rs;
   	rs.Open();
   	rs.AddNew();
   		
           rs.m_f1_tinyint = 255;
           rs.m_f1_smallint = 32484; // equivalent hex 0x7EE4
           rs.m_f1_int = 1246576928; // equivalent hex 0x4a4d4120L
           rs.m_f1_real = -9.123e19f;
           rs.m_f1_float = -9.123e19;
          
   	rs.Update();
   	rs.Close();
   	MessageBox("Updated"); 




7. Compile and run the code. Try inserting these values into the SQL Server database. Nothing is inserted into the SQL Server database because these values are treated as Pseudo Nulls.
   

Work Around

        CTestSet rs;
        rs.Open();	
	rs.AddNew();		        
 
	rs.m_f1_tinyint = 255;
	rs.SetFieldDirty(&rs.m_f1_tinyint);
	rs.SetFieldNull(&rs.m_f1_tinyint, FALSE);
    
	rs.m_f1_smallint = 32484; // equivalent hex 0x7EE4
         rs.SetFieldDirty(&rs.m_f1_smallint);
	rs.SetFieldNull(&rs.m_f1_smallint, FALSE);
	
	rs.m_f1_int = 1246576928; // equivalent hex 0x4a4d4120L
         rs.SetFieldDirty(&rs.m_f1_int);
	rs.SetFieldNull(&rs.m_f1_int, FALSE);
	
	rs.m_f1_real = -9.123e19f;        
	rs.SetFieldDirty(&rs.m_f1_real);
	rs.SetFieldNull(&rs.m_f1_real, FALSE);
	
	rs.m_f1_float = -9.123e19;       
	rs.SetFieldDirty(&rs.m_f1_float);
	rs.SetFieldNull(&rs.m_f1_float, FALSE);
		
		
	rs.Update();	
	rs.Close();	
	MessageBox("Updated"); 
 

Additional query words:

Keywords          : kbDatabase kbMFC kbSQLServ kbVC kbGrpVCDB 
Version           : winnt:5.0,6.0
Platform          : winnt 
Issue type        : kbprb 

Last Reviewed: June 23, 1999