OLE DB - First steps

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Introduction

This article intends to introduce you to using OLE DB for database access. It does this by showing you how to use OLE DB to insert records into a database and then read back those records. Before you start there are some things you need to do first, like creating an MDB file.

Things to do first...

• First create a new MS Access database called test.mdb and create a single table and call it 'main'.
• Now add two fields to 'main' called 'Name' of type 'Text' and 'Age' of type 'Number'.
• Copy test.mdb to d:\

Some concepts

In .NET, connections to databases and queries are achieved through data providers. The OLE DB .NET data provider is implemented through various classes within the System::Data::OleDb namespace. In this article we only examine three of these classes - OleDbConnection, OleDbCommand and OleDbDataReader. The OleDbConnection object represents a database connection. The OleDbCommand object wraps an SQL command that is performed on a database connection.  When we are making an INSERT or an UPDATE query on a database table, those two are the only classes we'll need. But when we are retrieving data from a table, we'll also need to use the OleDbDataReader class. This class allows us to browse through a row of records in a forward-only direction.

Inserting records code snippet

//Create the OleDbConnection object 
//and associate it with our database
OleDbConnection* conn = new OleDbConnection(
    "PROVIDER=Microsoft.Jet.OLEDB.4.0;Data Source=d:\\test.mdb");

//Open the database connection
conn->Open();

//Create an OleDbCommand object and
//pass it the SQL command and the OleDbConnection
//object to use to connect to the database
OleDbCommand* cmd = new OleDbCommand(sqlstr,conn);

//Execute the SQL command
cmd->ExecuteNonQuery();

//Close the connection to the database
conn->Close();

Inserting records is the simpler of the two processes. We create a connection using the OleDbConnection object, create an OleDbCommand object and associate it with the OleDbConnection object. Now we call the ExecuteNonQuery method, which will execute the SQL command we had passed to the OleDbCommand  constructor. We then close the connection.

Reading records code snippet

//Create the OleDbConnection object 
//and associate it with our database
OleDbConnection* conn = new OleDbConnection(
    "PROVIDER=Microsoft.Jet.OLEDB.4.0;Data Source=d:\\test.mdb");

//Open the database connection
conn->Open();

//Create an OleDbCommand object and
//pass it the SQL read query and the connection to use
OleDbCommand* cmd = new OleDbCommand(sqlstr,conn);

//Procure the OleDbDataReader object to browse the recordset 
OleDbDataReader* rdr = cmd->ExecuteReader();

//Keep reading records in the forward direction
while (rdr->Read())
{
   //Use one of the various methods available to read the data
   //Eg:- GetValue, GetValues, Item etc.
    . . .
    . . .    
}

//Close the connection to the database
conn->Close();

This is basically the same as far as creating the OleDbConnection and OleDbCommand  objects are concerned. But instead of calling ExecuteNonQuery directly, we call ExecuteReader which will return a OleDbDataReader object. We can use this OleDbDataReader object to browse through the recordset. Keep calling Read which will return false when it has finished the whole recordset. There are several ways to read from a recordset but I prefer get_Item which allows you to specify a field name opposed to other functions like GetValue which require us to pass the index of the field in the table which is a bad method in my opinion.

Console::WriteLine(rdr->get_Item("FullName"));

There is a sequel to this article on the use of bound controls with OLE DB which you can find here.

Revision History

• Jul 04 2002 - Did a full redo of the article, added a sample project and now uses MC++ instead of C#
• Dec 02 2001 - Changed the program for .Net beta 2.0

This article, along with any associated source code and files, is licensed under The Code Project Open License (CPOL)