title: C# Data Table Rows value is empty string, but there is Null value
body: <p>The value of dt.Rows.Count always equals 1, and the value of dt.Rows[0][0] and dt.Rows[0][ 1] is empty string when I query sqlStr of no records. </p>
  
  <p>But I execute sqlStr in database. It's null. <a href="http://i.stack.imgur.com/5mzgN.png" rel="nofollow">Here is the database screenshot</a>
  I don't know why it is.</p>
  
  <pre><code>string sqlStr = "select SUM(userIncome),SUM(userSpend) from tb_billlist where userPhone = '" + userPhone + "' and " +
       "billTime between '" + beginDate + "' and '" + endDate + "'";
  //execute sql
  DataTable dt = new DataTable();
  dt = MysqlHelper.ExecuteDataTable(sqlStr);
  if (dt.Rows.Count &gt; 0) 
  {
      userBillInfo.userAllIncome = (float.Parse)(dt.Rows[0][0].ToString());
      userBillInfo.userAllSpend = (float.Parse)(dt.Rows[0][1].ToString());
  }
  else
  {
      userBillInfo.userAllIncome = 0;
      userBillInfo.userAllSpend = 0;
  }
  </code></pre>