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Executing Multiple Firect ODBC SQL Calls

Friday Mar 1st 2002 by Dale Harkness

Have you ever tried to issue multiple direct ODBC calls only to have some of the calls after the first fail?

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Environment: VC++, SQL Server

Have you ever tried to issue multiple direct ODBC calls only to have some of the calls after the first fail? I encountered this problem on a project I was working on. The first SQL command would complete successfully but some of the following SQL commands would fail. The error produced indicated that an "Invalid cursor state" existed. The message provided me with no clues to what the nature of the problem was. The following sample shows code that fails producing the "Invalid Cursor State" message. The UpdateUserName() function contained the commands that produced the error. The sample was issued to an MS SQL Server running version 6.5:

//
// Change a given users name in the SQL database
//
void CSqlConnector::UpdateUserName( const char * pSysAdmnPwd,
                                    const char *pLogin, 
                                    const char *pNewUserName,
                                    const char *pOldUserName )
{
      CString SqlCommand;
      
      try
      {
            // a DSN has been created earlier named MyDSN 
            // that points to the SQL server 
            AllocateODBCHandles( "MyDSN", "sa", pSysAdmnPwd );

            // issue a direct SQL call (call a system 
            // procedure in this case)
            SqlCommand.Format( "sp_dropuser %s", pOldUserName );
            ExecuteDirectODBC( m_hStmt, SqlCommand );
 
//
// NOTE: this command works without using the allocation/
//       deallocation process
//
            // switch to the user database
            SqlCommand.Format( "USE %s", "MyDatabaseName" );
            ExecuteDirectODBC( m_hStmt, SqlCommand );
 
//
// THIS STATEMENT FAILED FOR ME
//
            // add the user with the new name
            SqlCommand.Format( "sp_adduser %s, %s",
                               pLogin,
                               pNewUserName ); 
            ExecuteDirectODBC( m_hStmt, SqlCommand );
 
 
            // Release the ODBC handles because
            // the SQL commands have completed  
            FreeODBCHandles();
      }
      catch(...)
      {
            // handle error condition
      }
}
 
//
// directly execute an ODBC command
//
SQLRETURN CSqlConnector::ExecuteDirectODBC( SQLHSTMT hStmt,
                                            CString command )
{
  //
  // issue the SQL command that is to execute
  //
  return SQLExecDirect(hStmt, 
       (unsigned char *)command.operator LPCSTR(), SQL_NTS);
}
 
//
//
void CSqlConnector::AllocateODBCHandles( const CString & DSN,
                                         const CString & UID,
                                         const CString & Pwd )
{
   try
   {
      // Allocate the Environment Handle
      SQLAllocHandle( SQL_HANDLE_ENV, 
                      SQL_NULL_HANDLE,
                      &m_hEnv );
 
      // Notify ODBC that this is an ODBC 3.0 application.
      SQLSetEnvAttr(m_hEnv, 
                    SQL_ATTR_ODBC_VERSION,
                    (SQLPOINTER) SQL_OV_ODBC3,
                    SQL_IS_INTEGER);
 
      // Allocate the Connection handle
      SQLAllocHandle( SQL_HANDLE_DBC, 
                      m_hEnv, 
                      &m_hDbc );
 
      // set the connection timeout value to 15 seconds
      SQLSetConnectOption(m_hDbc, SQL_LOGIN_TIMEOUT, 15);
 
      // Connect to the database 
      SQLConnect(m_hDbc, 
                (SQLCHAR*) DSN.operator LPCSTR(),
                SQL_NTS,
                (SQLCHAR*) UID.operator LPCSTR(),
                SQL_NTS,
                (SQLCHAR*) Pwd.operator LPCSTR(),
                SQL_NTS);
 
      // retrieve a statement handle
      SQLAllocHandle( SQL_HANDLE_STMT,
                      m_hDbc,
                      &m_hStmt );
   }
   catch(...)
   {
      // free the database handle if it exists
      if( NULL != m_hDbc )
      {
         // close the connection handle
         SQLDisconnect( m_hDbc );
      }
 
      // free the environment handle if it exists
      if( NULL != m_hEnv )
      {
         // free the environment handle
         SQLFreeHandle( SQL_HANDLE_ENV,
                        m_hEnv );
      }
   }
}
 
//
//
//
void CSqlConnector::FreeODBCHandles()
{
      // free the statement handle
      SQLFreeHandle( SQL_HANDLE_STMT, m_hStmt );
      // free the environment handle
      SQLFreeHandle( SQL_HANDLE_ENV, m_hEnv );
      // close the connection handle
      SQLDisconnect( m_hDbc );
      // free the connection handle
      SQLFreeHandle( SQL_HANDLE_DBC, m_hDbc );
 
      // mark the handles as not in use
      m_hEnv = NULL; 
      m_hDbc = NULL; 
      m_hStmt = NULL; 
}

I checked the newsgroups for anyone having the same problem. I found several posts where others encountered the same problem. No answers were posted to their questions. After trying a number of angles I finally stumbled on one that worked. The code worked for me when I allocated the ODBC handles before each command and then deallocated them after each call. The following is a snippet code that demonstrates this method:

//
// Change a given users name in the SQL database
//
void CSqlConnector::UpdateUserName(const char * pSysAdmnPwd,
                                   const char *pLogin,
                                   const char *pNewUserName,
                                   const char *pOldUserName )
{
   CString SqlCommand;
   
   try
   {
      // a DSN has been created earlier named MyDSN that
      // points to the SQL server 
      AllocateODBCHandles( "MyDSN", "sa", pSysAdmnPwd );
 
      // add the login
      SqlCommand.Format( "sp_dropuser %s", pOldUserName );
      ExecuteDirectODBC( m_hStmt, SqlCommand );
 
      // Release the ODBC handles because
      // the SQL command has completed  
      FreeODBCHandles();
 
      AllocateODBCHandles( "MyDSN", "sa", pAdminPwd ))
 
      // switch to the user database
      SqlCommand.Format( "USE %s", "MyDatabaseName" );
      ExecuteDirectODBC( m_hStmt, SqlCommand );
 
//
// THE COMMAND SUCCEEDS THIS TIME
//
      // add the user with the new name
      SqlCommand.Format( "sp_adduser %s, %s",
                         pLogin, 
                         pNewUserName ); 
      ExecuteDirectODBC( m_hStmt, SqlCommand );
 
      // Release the ODBC handles because
      // the SQL commands have completed  
      FreeODBCHandles();
   }
   catch(...)
   {
      // handle error condition
   }
}

I've noticed that some statements would work successfully without the allocation/deallocation process. It appears that certain statements need to be "flushed" before other commands may be issued. Closing and opening the ODBC handles appears to handle this. I hope this article helps clear up the same problem for others.

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