C#. When to use using (in db)?

Question

Database

using (SqlConnection con = new SqlConnection())
{
     SqlCommand cmd = con.CreateCommand();

     cmd.CommandText = "SELECT * FROM Tab";

     using (SqlDataReader reader = cmd.ExecuteReader())
     {
          // reading...
     }
}

And also SqlCommand has a Dispose method , which means I can wrap cmd in using . Do I need to do all this? Or is it enough to put using on one thing???