How to retrive data from database by C#

//First we should add the following Namespace

using System.Data;
using System.Data.OleDb;


{
OleDbConnection connection = new OleDbConnection("Provider=Microsoft.Jet.OLEDB.4.0;Data Source=|DataDirectory|\\ERDATA.mdb;Persist Security Info=True");
        connection.Open();
        String query = "Select sum(amount) FROM Tran";
        OleDbDataAdapter dadaptor = new OleDbDataAdapter(query, connection);
        DataSet ds = new DataSet();
        dadaptor.Fill(ds);

        if (ds.Tables[0].Rows.Count > 0)
        {
            String sum = ds.Tables[0].Rows[0][0].ToString();
            lblTotal.Text = "Rs. " + sum;
        }
        connection.Close();
        connection.Dispose();
}