关于Fill: SelectCommand.Connection 属性尚未初始化

string cmd = "select * from mydb";
SqlCommand sql = new SqlCommand(cmd);
SqlConnection conn = new SqlConnection(@"Data Source=Localhost;Database=company;Integrated Security=True");
try
{

            conn.Open();
            SqlDataAdapter da = new SqlDataAdapter();

            DataSet ds = new DataSet();
            da.SelectCommand = sql;
            da.Fill(ds);
            dataGridView1.DataSource = ds.Tables[0];
        }

                        da.Fill(ds);这里提示错误：Fill: SelectCommand.Connection 属性尚未初始化。