CODBCRecordset class

CODBCRecordset class is intended to be full replacement of all ClassWizard generated CRecordset derived classes in MFC projects.

This article was contributed by Stefan Tchekanov.
Platform: VC 5.0 & VC 6.0

 

Introduction

In a usual MFC database project we have a lot of CRecordset derived classes generated by ClassWizrad. The presented here CODBCRecordset class is a very easy to use replacement of all these CRecordset classes. CODBCRecordset is derived from CRecordset but it does not have hardcoded inside the number and the type of the database fields.

I have seen some implementations trying to solve this problem. But CODBCRecordset has at least two advantages:

  1. CODBCRecordset can be used not only to get values from the database, but to write values into the database too using the MFC way.
  2. The other implementations canot open simultaneously more than one recordset trough one database connection when the database is MS SQL Server. In some cases this could be very big problem because opening a new connection is time and resources expensive.

Because CODBCRecordset is derived from CRecordset and uses its data exchange mechanism it is fully compatible with MFC ODBC class CDatabase.

Each field value is stored in a CDBField object. CDBField class is inherited from CDBVariant and has added some extra functionality to be easy to use.

CODBCRecordset and CDBField classes support all database data types. CDBField makes implicit type conversion where is appropriate to supply the data in the requested format.

Here is a list of CODBCRecordset and CDBField methods:

CODBCRecordset class
Constructor The same as for CRecordset accepting CDatabase*
BOOL Open( LPCTSTR lpszSQL,
                      UINT nOpenType,
                      DWORD dwOptions );		 Open the recordset
lpszSQL is a SQL statement that returns recordset
e.g. SELECT * FROM tablename
nOpenType is CRecordset open type, see CRecordset::Open()
dwOptions is CRecordset options, see CRecordset::Open()

Note that lpszSQL and nOpenType have exchanged their positions compared to CRecordset::Open()
short       GetODBCFieldCount() Returns number of the fields (columns) in the recordset. This method is defined in CRecordset.
int            GetFieldID( LPCTSTR ) Returns a field index by given field name. It is case insensitive. CRecordset::GetFieldIndexByName() works, but is case sensitive.
CString   GetFieldName( int ) Returns a field name by given field index
CDBField&   Field( LPCTSTR )
CDBField&   Field( int )		 Through this method you can get a reference to the internal CDBField object corresponding to the specified column (See the CDBField class table for details about CDBField).

There are two forms of the method - with argument of type LPCTSTR szName - specifying the name of the column, and int nID - specifying the index of the coulmn in the recordset.

These methods can be used as lvalue in expressions. Thus you can write values to the database.
CDBField&   operator( LPCTSTR )
CDBField&   operator( int )		 The function operator is defined for easy of use of the class and just calls the corresponding Field() method

There are two forms of the function operator - with argument of type LPCTSTR szName - specifying the name of the column, and int nID - specifying the index of the coulmn in the recordset.

These can be used as lvalue in expressions. Thus you can write values to the database.
bool GetBool()
unsigned char GetChar()
short GetShort()
int GetInt()
long GetLong()
float Getfloat()
double GetDouble()
COleDateTime GetDate()
CString GetString()
CLongBinary* GetBinary()
All of these methods do the appropriate type conversions depending on their return value and the type of the underlying data in the database.

These methods just call Field().AsXXX()
(See the CDBField class table for details about CDBField).

There are two forms of these methods - with argument of type LPCTSTR szName - specifying the name of the column, and int nID - specifying the index of the coulmn in the recordset.

These cannot be used as lvalue in expressions.

 

CDBField class
Constructors No public constructors. CDBField cannot be instantiated except by CODBCRecordset to be used in internal structures to support data exchange. These objects are accessible through CODBCRecordset methods.
bool AsBool()
unsigned char AsChar()
short AsShort()
int AsInt()
long AsLong()
float AsFloat()
double AsDoble()
COleDateTime AsDate()
CString AsString()
CLongBinary* AsBinary()
All of these methods do the appropriate type conversions depending on their return value and the type of the underlying data in the database (See the AsXXX methods table for data conversion rules).

There is no data type Int but AsInt() is equal to AsLong()
assignment operators There are defined assignment operators accepting bool, unsigned char, short, int, long, COleDateTime and CString. So CDBField objects can be lvalues. There is no assignemt operator accepting CLongBinary because MFC does not support writing CLongBinary values into database. These assignment operators do appropriate conversions to the underlying database column data type except CLongBinary (See the assignment operators table for data conversion rules).
const    CString&    GetName() Returns the field name this object corresponds to.
bool IsNull()
bool IsBool()
bool IsChar()
bool IsShort()
bool IsInt()
bool IsLong()
bool IsFloat()
bool IsDouble()
bool IsNumber()
bool IsDate()
bool IsString()
bool IsBinary()
Each of these return true if the field contains a value of the corresponding data type.

There is no data type Number but IsNumber() returns true if IsShort() || IsLong() || IsFloat() || IsDouble().

There is no data type Int but IsInt() returns true if IsLong() returns true.

 

Conversions made by AsXXX methods
 

Values in the database
  NULL BOOL UCHAR SHORT LONG SINGLE DOUBLE DATE STRING BINARY
AsBool false * * * * * *   *  
AsChar 0x32 * * * * * *   *  
AsShort 0 * * * * * *   *  
AsInt 0 * * * * * *   *  
AsLong 0 * * * * * *   *  
AsFloat 0.0 * * * * * *   *  
AsDouble 0.0 * * * * * *   *  
AsDate null invalid invalid * * * * * *  
AsString empty * * * * * * * * *
AsLongBinary NULL                 *
Empty cells indicate the conversion is not available, thus code asserts.
Cells marked with * indicate conversion is available (See the Conversion algorithms table for data conversion rules).

 

Conversions made by assignment operators
Database Field Type

Argument of the assignment operator
  bool unsigned char short int long float double COleDateTime String
NULL                  
BOOL * * * * * * *   *
UCHAR * * * * * * *   *
SHORT * * * * * * *   *
LONG * * * * * * *   *
SINGLE * * * * * * *   *
DOUBLE * * * * * * *   *
DATE               * *
STRING * * * * * * * * *
BINARY                  
Empty cells indicate the conversion is not available, thus code asserts.
Cells marked with * indicate conversion is available (See the Conversion algorithms table for data conversion rules).

 

Conversion algorithms
String to Bool comparing the first character of the string with 'T'
Char to Bool comparing the character with 'T'
Bool to String String = (bVal) ? 'T' : 'F'
Bool to Char Char = (bVal) ? 'T' : 'F'
String to Number appropriate atoX() function
Number to String CString::Format() method using the appropriate format specifier string
String to Date COleDateTime::ParseDateTime() method
Date to String COleDateTime::Format() method

 

Examples of how to use CODBCRecordset

You should include the files ODBCRecordset.h and ODBCRecordset.cpp in your project.

I usually include this line in my StdAfx.h file.

#include "ODBCRecordset.h"

Here is a simple code showing how CODBCRecordset can be used.

/////////////////////////////////////////////////////////////////////////////
CDatabase	db;
//	This connect string will pop up the ODBC connect dialog
CString		cConnect = "ODBC;";
db.Open( NULL,				//	DSN
	 FALSE,				//	Exclusive
	 FALSE,				//	ReadOnly
	 cConnect,			//	ODBC Connect string
	 TRUE				//	Use cursor lib
   );

COleDateTime	dOrderDate;

CODBCRecordset	rs( &db );
rs.Open( "SELECT * FROM Orders \
	WHERE ORDER_DATE > 'jan 1 2000' \
	ORDER BY ORDER_DATE" );
for( ; ! rs.IsEOF(); rs.MoveNext() )
{
//	The choice is yours. You may choose whatever way
//	you want to get the values
//
	//	These return COleDateTime value
	dOrderDate = rs.GetDate( "ORDER_DATE" );
	dOrderDate = rs.Field("ORDER_DATE").AsDate();

	//	These make implicit call to AsDate()
	dOrderDate = rs("ORDER_DATE");
	dOrderDate = rs.Field("ORDER_DATE");

	//	Now edit the fields in the recordset
	rs.Edit();
	rs("ORDER_DATE") = "jan 1 1999";	// Implicit conversion
	rs.Field("ORDER_DATE") = "jan 1 1999";  // Implicit conversion
	rs.Update();
}	//	for(....
/////////////////////////////////////////////////////////////////////////////

If ORDER_DATE is stored in the database as datetime or compatible data type the value will be get directly.
If ORDER_DATE is stored in the database as string or compatible data type (char, varchar) the value will be converted via COleDateTime::ParseDateTime() method. If conversion fails, dOrderDate will be set to COleDatetime::invalid.

When opening a resultset generated by join statements it is possible to get 2 or more columns that have the same name. CODBCRecordset leaves the name of the first column intact but other repeated columns are renamed with adding the number this columns repeats the name. Not repeated column names are left intact. E.g.

SELECT * FROM Orders, Customers WHERE Orders.CUST_ID = Customers.ID

If the table Orders have a column with name ID and Customers have a column with name ID, CODBCRecordset will rename ID from Customers to ID2 and all other not repeating column names will be intact.

Well, here is a tip: Rename columns manualy to be sure what name they have, e.g.

SELECT Orders.*, Customers.ID as CUSTOMERS_ID
    FROM Orders, Customers
    WHERE Orders.CUST_ID = Customers.ID

 

How does CODBCRecordset work?

CODBCRecordset allocates storage for all fields in the resultset and uses MFC Record Field eXchange mechanizm like it has been inherited from CRecordset using ClassWizard.

That's all.

Download source   ODBCRecordset.zip (13 KB)