数据库读取某一列

private List Get_RoleName(int id)
        {
            SqlConnection conn = GetSqlConnection();
            SqlCommand cmd = conn.CreateCommand();
            string sql = "select R_RoleName from sys_Roles where RoleID in(select R_RoleID from sys_UserRoles where R_UserID=" + id + " )";
            cmd.CommandText = sql;
            conn.Open();
            SqlDataReader reader = cmd.ExecuteReader(CommandBehavior.CloseConnection);
            List lst = new List();
            while (reader.Read())
            {
                lst.Add(reader.GetString(0));
            }
            return lst;
        }