14 Database Management With Microsoft ODBC

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Chapter 14

Database Managementwith Microsoft ODBC

Objectives Describe the role of ODBC in the applications that interact with databases.

Explain ODBC architecture.

List the various classes involved in database applications.

Access data from different DBMSs using the MFC classes

Implement an ODBC connection to a local database.

Advantages of using DAO classes

IntroductionA database management system (DBMS) consists of a collection of interrelated data and set of

programs to access that data. Database systems are designed to manage large bodies of

information. It provides mechanisms for defining structures for the storage of information, efficient

manipulation of data and information retrieval. This chapter gives you a brief description of DBMS

and its advantages.

Visual C++ provides several ways to work with databases. Besides writing stand-alone database

applications, you can often use a database effectively in other kinds of programs as a convenient

storage and retrieval medium.

ODBC is a call-level interface that allows applications to access data in any database for whichthere is an ODBC driver. Using ODBC, you can create database applications with access to any

database for which your end-user has an ODBC driver. ODBC provides an API that allows your

application to be independent of the source database management system (DBMS).

ODBC data sources are accessible through the MFC ODBC classes, as described in this chapter.

Advantages of using DBMSUse of standard file formats - Popularly the database files are accepted in the format of .dbf or

Microsoft .mdb.

Indexed file access - For quick access to records by key, you need an indexed file. You can

write your own B-tree file access routines, but thats a tedious job which has been done already.

All DBMSs contain efficient indexed access routines.

Data integrity safeguards - Many DBMSs come with security systems for protecting their data.

One example is transaction processing. A transaction encompasses a series of related changes.

If the entire transaction cant be processed, it is rolled back so that the database reverts to its

original state before the transaction.

Multi-user access control - Your application may need multi-user access. Most DBMSs provide

record locking to prevent interference between simultaneous users.


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Page 172 Database Management with Microsoft ODBC

Open Database Connectivity (ODBC) ODBC Architecture

MFC ODBC Classes

Handling Transactions in ODBC

When DBMS was first introduced, applications typically used embedded SQL to access a specific

DBMS. The application used to have a pre compiler that converted the SQL statements into data

access code for that particular DBMS. The venders of the next DBMS generation started

providing DLLs for APIs that allow an application to compile SQL statements at run time. The

Open Database Connectivity (ODBC) is a new approach in DBMS APIs. It is an interface that

enables applications to access multiple, heterogeneous data sources. It provides common APIs

that is capable of accessing all the major DBMSs such as Oracle, SQL server, etc.

The ODBC provides the following:

1) A library of ODBC function calls that allow an application to connect to a DBMS, execute SQL

statements and retrieve the results.

2) A standard set of error codes.

3) The ODBC standard defines not only the rules for SQL grammar but also the C-language

programming interface to a SQL database.

4) It is now possible for a single compiled C or C++ file to access the DBMS databases having

the ODBC drivers.

5) The ODBC Software development kit (SDK), included with Visual C++, contains 32-bit drivers

for DBF files, Microsoft Access databases etc.

6) Other database companies, including Oracle, Informix, Ingres, Gupta provide their own ODBC

drivers for their DBMS.

7) You no longer have to buy the interface tool from the same company that supplies you the

database engine.

The ODBC Architecture

ODBC has a unique DLL - based architecture that makes the system completely modular.

Any typical ODBC application has four main components in its architecture.

1) The application

2) ODBC driver manager

3) ODBC driver

4) Actual data source

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Data Data

Oracle Server(Data Source)

SQL Server(Data Source) Data

Application

Driver Manager

Oracle

Driver

SQL

ServerDriver

dBaseDriver


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Database Management with Microsoft ODBC Page 173

In ODBC, the data source name defines a set of data, with the information required to access

that data and the location of the data source. (You can define a data source name from the

ODBC administrator in the Windows Control Panel). A data source is a specific instance of

data hosted by some database management system (DBMS).

The application performs the processing and calls ODBC functions to submit SQL statements

and receive results.

The ODBC driver manager loads and unloads the application requested drivers along with

processing some of the function call.

The ODBC driver processes most of the function calls, submits SQL requests to a specific

data source and returns results to the application.

The data source consists of data the application is accessing. This may be a DBMS or other

type of data such as dBASE or FoxPro file.

A small top-level DLL, ODBC32.DLL defines the API. ODBC32.DLL calls database specific

DLLs , known as drivers during the program execution.

With the help of windows registry (maintained by the ODBC Administrator module in the

Windows Control Panel) ODBC32.DLL tracks which database - specific DLLs are available

and thus allows a single program to access data in several DBMS simultaneously.

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The MFC ODBC Classes

MFC contains all the components you will need to write C++ database applications for

Windows.

MFC comes with two separate database access systems : ODBC (Open Database

Connectivity) and DAO (Data Access Objects).

The two principal ODBC classes are CDatabase and CRecordset. Objects of type

CDatabase represent ODBC connections to data sources and objects of type CRecordsetrepresent the scrollable rowsets (usually snapshots).

Recordset is a block of data containing records (may be returned by an SQL query). The

CRecordset object contains the current record in the recordset. Snapshot is a static recordset,

i.e. the changes in the database are not reflected in the recordset. Dynaset is a recordset,

which reflects the changes (updates, not additions or deletions), in the database.

You seldom derive classes from CDatabase but you always derive classes from CRecordset

to match the columns in your database tables.

CRecordset

This class represents one row (record) in the recordset. This class along with the ODBCcode manages the dynaset or snapshot. Recordsets enable scrolling from record to

record, updating records (adding, editing, and deleting records), qualifying the selection

with a filter, sorting the selection, etc.

Typically, your program should derive a class from CRecordset (with the help of

AppWizard or ClassWizard ) and create an instance of the class . Now it should call the

inherited Open member function. Using the values of the parameters and data members

CRecordset constructs and opens a CDatabase object, it issues an SQL select statement

and then moves to the first record. Your program would then call other CRecordset

member functions to position the cursor and exchange data between database fields and

the members in your class representing the fields. When the object derived from your new

class is deleted, the recordset is closed, and the connection is released.

The simplest way to customize a query is to specify a WHERE clause (m_strFilter) and /or an ODER BY clause (m_strSort) before creating the recordset. To take advantage of

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MFC Database

Application Program

ODBC32.DLL

Driver Manager

ODBCJT32.DLL

Jet Controller

ODBCCR32.DLL

cursor library

MSJT3032.DLL

Jet DataBase Engine

SQL Server

ODBC Driver

SQL Server

Local DBF Files Local MDB filesRemote Shared

Databases

MSXB3032.DLL

Xbase driver


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filtering and sorting the recordset, set the desired values for m_strFilterand m_strSort

before calling OnInitialUpdate ofCRecordView

For filtering

m_pSet ->m_strFilter = [PubID] = + strUserDefinedID;

For sorting

m_pSet ->m_strSort = Title ASC ;

CRecordset Overridables

DoFieldExchange Called to exchange data (in both directions) between the field data

members of the recordset and the corresponding record on the data source. Implements

record field exchange (RFX).

GetDefaultConnect Called to get the default connect string.

GetDefaultSQL Called to get the default SQL string to execute.

OnSetOptions Called to set options for the specified ODBC statement.

OnWaitForDataSource Called to yield processing time to other applications for

asynchronous operations.

You can join two database tables using ClassWizard.

Use ClassWizard to access the class derived from CRecordset. Click the update

columns buttons to select the table to be joined. The Bind All button will add data

member to the new fields.

Edit the GetDefaultSQL function of the recordset to access two tables.

Modify the filter string m_strFilter.

CDatabase

Before an application can interact with the database, it must request a connection with a

data source. The CDatabase class represents a connection to a data source, through

which the application operates on the data source. You can have one or more CDatabase

objects active at a time in your application.

To use CDatabase, construct a CDatabase object and call its Open member function.

This opens a connection. When you then construct CRecordset objects for operating on

the connected data source, pass the constructor of the CRecordset object a pointer to

your CDatabase object. When you finish using the connection, call the Close member

function and destroy the CDatabase object. Close closes any recordsets you have not

closed previously.

Database Overridables

OnSetOptions Called by the framework to set standard connection options. The default

implementation sets the query timeout value and the processing mode (asynchronous or

synchronous). You can establish these options ahead of time by calling

SetQueryTimeout and SetSynchronousMode.

OnWaitForDataSource Called by the framework to yield processing time to other

applications during a lengthy operation.

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Fig : MFC ODBC class database relationship

CRecordView

Provides a form view directly connected to a recordset object. The dialog data exchange(DDX) mechanism exchanges data between the recordset and the controls of the record

view. Like all form views, a record view is based on a dialog template resource. Record

views also support moving from record to record in the recordset, updating records, and

closing the associated recordset when the record view closes. The application created by

using AppWizard with Database view option provides toolbar buttons that enable the user

to step forward and backword through a database table. The CRecordView class is a

form view class that is attached to a recordset.

Like CFormViewclass, the CRecordViewclass depends on a dialog template.

The CFormView class has data members that correspond to the controls in the

Dialog, but the CRecordViewclass accesses objects in a foreign object, namely the

attached CRecordsetobject.

When the user enters data in the controls, the record views DDX(Dialog DataExchange) code moves the data into the recordset data members , which are bound

to the database columns by the recordsets RFX(Record Field Exchange) code.

To write a simple application using CRecordview

1) Create a new workspace with SDI option.

2) In the database support dialog box, select Database view without file option.

3) Press the Data Source button to invoke the Database options dialog box. In this

select the appropriate data source.

4) Now the AppWizard will create two classes, one derived from CRecordset and one

derived from CRecordView ( Which has as its member a pointer to the class derivedfrom CRecordset ). It will also generate an empty dialog template. At runtime, the

recordview object and the recordset objects are connected. You should add controls

to the dialog template and match the controls to recordset data members.

If you are accessing the FoxPro Database, you dont have to write anything for updating

the Database. But if you are accessing the Access Database, the AppWizard will create a

Read Only view based database application. Since all the scrolling and default updating

is by CRecordView::OnMove function , you should override this function , if you want to

add, delete or update records from your application. Also you should add the code to add

(AddNew), delete (Delete) and update (Edit) records. You should call Requeryto see the

changes.

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CAuthorSet object

(embedded in document)

m_pDatabase m_hstmnt

ODBCrowset

Dynaset or

snapshot

Database

CDatabase object

m_hdbc

ODBC connection


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Fig : Relationship between the ODBC database objects and data exchange machanism.

E.g. Consider a database for a student record with the fields Name, Roll_No and

Total_Marks.

Create an application using AppWizard according to above given steps. Now to make it a

Read-Write application, you will have to override the OnMove function of CRecordView

os follows :

BOOL COdbcView : : OnMove(UINT nIDMoveCommand)

{

switch( nIDMoveCommand )

{

case ID_RECORD_PREV :

m_pSet->MovePrev();

if( ! m_pSet->IsBOF( ))

break;

case ID_RECORD_FIRST :

m_pSet->MoveFirst();

break;

case ID_RECORD_NEXT:m_pSet->MoveNext();

if( ! m_pSet->IsEOF( ) )

break;

if( m_pSet->CanScroll ( ) )

{

m_pSet->SetFieldNull ( NULL );

break;

}

case ID_RECORD_LAST:

m_pSet->MoveLast( ) ;

break;default:

ASSERT( FALSE );

}

UpdateData( FALSE );

return TRUE;

}

Edit the menu command handlers as follows:

void COdbcView :: OnRecordAdd()

{ m_pSet->AddNew();

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Name :

Room :

MyRecordSet

{

m_strName;

m_iRoomNos

}

NameRoll No.

Record View

RecordSetDatabase Table

RFX

DDX


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UpdateData(TRUE);

if(m_pSet->CanUpdate())

{

m_pSet->Update();

}

m_pSet->ReQuery();UpdataData(FALSE);

}

void COdbcView :: OnRecordClearFields()

{

m_pSet->Name = _T();

m_pSet->Total_Marks = 0;

m_pSet->Roll_No = 0;

UpdataData(FALSE);

}

void COdbcView :: OnRecordDelete()

{

m_pSet->Delete();

m_pSet->MoveNext();

// back to last if we moved off the end

if(m_pSet->IsEOF()) {

m_pSet->MoveLast();

}

//Is the set now empty

if(m_pSet->IsEOF()) {

m_pSet->SetFieldNull(NULL);

}

UpdateData(FALSE);

}void COdbcView :: OnRecordUpdate()

{

m_pSet->Edit();

UpdateData(TRUE);

if(m_pSet->CanUpdate())

{

m_pSet->Update();

}

m_pSet->ReQuery();

}

Other MFC classes supporting ODBC

CDBException - An exception resulting from failures in data access processing. This class

serves the same purpose as other exception classes in the exception-handling mechanism of

the class library.

CFieldExchange - Supplies context information to support record field exchange (RFX),

which exchanges data between the field data members and parameter data members of a

recordset object and the corresponding table columns on the data source. Analogous to class

CDataExchange, which is used similarly for dialog data exchange (DDX).

Related Class- CLongBinary Encapsulates a handle to storage for a binary large object (or

BLOB), such as a bitmap. CLongBinary objects are used to manage large data objects storedin database tables.

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Handling Transactions in ODBC

Transaction is a logical set of operations that are to be executed atomically. The database is

in consistent state before and after the execution of the transaction. If the transaction fails in

between, then the database is rolled back to its original state.

For example, to perform the simple task of giving someone a raise, you might select a current

salary, add 10% to it and then update it with the new account. You might also go back to their

personal record and adjust their tax, withholding information.

This is one transaction, even though it involves three SQL statements: one SELECT and two

UPDATE. If the SELECT works and the first UPDATE changes the persons salary, but the

subsequent tax UPDATE fails , it is very convenient to be able to rollback the transaction ,

scratching out any changes since the transaction was strated . If everything in the transaction

works correctly, the transaction can be committed

In MFC, you can rollback a transaction by calling Rollback() against the CDatabase object .

If you successfully complete a transaction, you can call CommitTrans() on the CDatabase

object to have a transaction permanently recorded by the database. For these calls to work,

you must call BeginTrans()against the database ; the first statement executed after your callto BeginTrans() is the first statement which will be undone if you call RollBack(), or actually

written if you call CommitTrans().

Multiple RecordSets

In the previous examples, the view object contained an embedded recordset that was created

with the CRecordset default constructor, which caused the creation of a CDatabase object.

The views OnInitialUpdate function called CRecordset::Open , which called the virtual

GetDefaultConnect function, opened the database, and then called the virtual GetDefaultSQL

function.

The problem with this scenario is that there can be only one recordset per database becausethe database is embedded in the recordset.

To get multiple recordsets, you have to do things a different way - you must create the

CDatabase object first.

Once youve done that, you can construct as many recordsets as you want, passing a

CDatabase pointer as a parameter to the CRecordset constructor.

You start by embedding a CDatabase object in the document in the place of the CRecordset

object.

You also include a pointer to the primary recordset.

Adding ODBC Capability to An MFC ApplicationAdd the following line at the end of StdAfx.h:

#include

Edit the RC file in text mode. After the line

#include afxprint.rc //printing/print preview

resources\r\n

Add the line

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#include afxdb.rc //database resources\r\c

And after the line

#include afxprint.rc //printing/print preview

Add the line

#include afxdb.rc //database resources

DAO DAO database classes in MFC.

Advantages of DAO classes.

Similarities Between ODBC Database Classes and MFC DAO Database Classes.

Data Access Objects (DAO) was added to MFC in version 4. It provides a framework for using

code to create and manipulate databases. DAO supplies a hierarchical set of objects that use theMicrosoft Jet database engine to access data and database structure in:

Database that use the Microsoft Jet (.MDB) databases engine

ODBC data sources, using an ODBC driver

Installable ISAM databases, such as

dBASE

Paradox and

Microsoft FoxPro

DAO database classes in MFC

MFC provides a set of classes that supports the DAO technology.

The following table lists the MFC DAO classes and the DAO objects they represent.

Class DAO object Description

CDaoWorkspace Workspace Manages transactions and access to

properties of the database engine. The

Workspace manages one or more

CDaoDatabase objects.

CDaoDatabase Database Manages a connection to a database

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CObject


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CDaoRecordset Recordset Manages a result set (recordset), a set

of records returned by a query.

Recordset can be tables, dynasets, or

snapshots.

CDaoRecordView (none) Enables you to view the contents of a

recordset in a CFormView derived

object

CDaoFieldExchange (none) Manages the exchange of data

between the recordset and the

database.

CDaoException Error Represents an exception condition

arising from the MFC database

classes based on DAO.

CDaoQueryDef Querydef Manages saving queries in a database

for reuse.

CDaoTableDef Tabledef Manages manipulating or viewing the

structure of a table in a database.

COleVariant VARIANT Used to pass data between the

application and the DAO object.

CByteArray (none) Used to transfer large binary objects

(like bitmaps) between a database and

the application.

Advantages of DAO classes

The DAO classes have the following advantages :

1) Better performance in some cases, particularly when using Microsoft Jet (.MDB)

databases.

2) Compatibility with the ODBC classes and with Microsoft Access Basic and Microsoft

Visual Basic.

3) Access to validation rules.

4) Ability to specify relations between tables.

5) A richer data access model, with support for Data Definition Language (DDL) as well as

Data Manipulation Language (DML).

Similarities Between ODBC Database Classes and MFCDAO Database Classes

The original design of the MFC ODBC classes was based on the DAO object model that has

been in use in Microsoft Access and Microsoft Visual Basic. This means that there are many

common features of the ODBC and DAO MFC classes. In general, the programming models

are the same.

A few similarities are:

1) Both the ODBC and DAO classes have database objects that manage using the

underlying database management system (DBMS).

2) Both have recordset objects representing a set of results returned from that DBMS.

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3) The DAO database and recordset objects have members nearly identical to the ODBC

classes.

4) With both sets of classes, the code to retrieve data is identical except for some object and

member name changes. Changes will be required, but usually the process is a

straightforward name change when switching from the ODBC classes to DAO classes.

5) For example in both models the procedure to retrieve data is to create and open adatabase object, create and open a recordset object, and navigate (move) though the

data performing some operation.

Note :In the DAO world, RFX is called DFX-short for DAO Record Field Exchange.

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Summary ODBC is an interface that enables applications to access multiple, heterogeneous

data sources.

ODBC application has following components Application code

ODBC API

ODBC driver manager

DBMS specific drivers

Recordset is a block of data containing records. The CRecordset object contains

the current record in the recordset.

Snapshot is a static recordset, i.e. the changes in the database are not reflected

in the recordset.

Dynaset is a recordset, which reflects the changes (updates, not additions or

deletions), in the database.

Query can be customized by specifying a WHERE clause (m_strFilter) and / or an

ODER BY clause (m_strSort) before creating the recordset.

DAO supplies a hierarchical set of objects that uses the Microsoft Jet database

engine to access data and database structures.

Quiz8) List the ODBC architectural components.

9) What is a recordset?

10) What is the difference between snapshot and dynaset?

11) Give two important data members of CRecordSet class?

12) Similarities between ODBC and DAO.

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14 Database Management With Microsoft ODBC

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