c# Asp.net multiple SQL

Problem: 1) I would like to add multiple SQL command, like this , how can i do?

command.CommandText = "select * from Department where Name ='" + TextBox1.Text + "' and Password ='" + TextBox2.Text + "'";

---

  OleDbConnection connection = new OleDbConnection();
    connection.ConnectionString = @"Provider=Microsoft.ACE.OLEDB.12.0;Data Source=C:\Users\sim\Desktop\Web.accdb";
    connection.Open();
   OleDbCommand command = new OleDbCommand();
     command.Connection = connection;
     command.CommandText = "select * from Registration where Name ='" + TextBox1.Text + "' and Password ='" + TextBox2.Text + "'";

    OleDbDataReader reader = command.ExecuteReader();
    int count = 0;

    while (reader.Read())
    {
       count = count + 1;
    }
    if (String.IsNullOrEmpty(TextBox1.Text))
    {
        MessageBox.Show("You have't input the Username.");
    }
    if (String.IsNullOrEmpty(TextBox2.Text))
    {
        MessageBox.Show("you havn't input the Password.");
    }
    if (count == 1)
    {

        Session["UserID"] = TextBox1.Text ;
        Response.Write("Username and password is valid");
        connection.Close();
        connection.Dispose();
        Response.Redirect("Purchase.aspx", true);

    }
    else
   {
        MessageBox.Show("Username or Password is not matched");
    }