sql查询两种写法

写法一：

            string sql = "select * from tbColumns";
            using (SqlConnection conn = new SqlConnection(PubVarData.ConnectionString))
            {
                conn.Open();
                SqlCommand cmd = new SqlCommand();
                cmd.Connection = conn;
                cmd.CommandText = sql;
                cmd.CommandType = CommandType.Text;
                using (SqlDataAdapter adapter = new SqlDataAdapter())
                {
                    adapter.SelectCommand = cmd;
                    DataSet ds = new DataSet();
                    adapter.Fill(ds);

                    string jSon = DataConvertHelper.convertDataTableToString(ds.Tables[0]);
                    return jSon;
                }
            }

            string sql = "select * from tbColumns";
            using (SqlConnection conn = new SqlConnection(PubVarData.ConnectionString))
            {
                conn.Open();
                using (SqlCommand cmd = new SqlCommand { Connection = conn, CommandText = sql, CommandType = CommandType.Text })
                {
                    using (SqlDataAdapter adapter = new SqlDataAdapter { SelectCommand = cmd })
                    {
                        using (DataSet ds = new DataSet())
                        {
                            adapter.Fill(ds);
                            string jSon = DataConvertHelper.convertDataTableToString(ds.Tables[0]);
                            return jSon;
                        }
                    }
                }
            }