获取数据集ds.Tables [0] .Rows.Count = 0

Question

Hi I am developing an application in VS 2010 C# and MySql

I have a function to validate is provided credentials are exist or not. Code is as below.

private bool Validate(string studentId, string time)
    {
        var msCon = _dal.OpenMySQLConnection();
        var da = new MySqlDataAdapter("SELECT * FROM tblStudent where Registeration_Date='" + time.Trim() + "' and studentId='" + studentId.Trim() + "' order by date_time desc limit 1", msCon);
        var ds = new DataSet();
        da.Fill(ds);
        var count = ds.Tables[0].Rows.Count;
        if (count == 0)
        {
            updateOrExist.Text = @"E";
            updationTime.Text = @"Current Checked Time: " +  DateTime.Now.ToShortTimeString();
            return true;
        }
        updationTime.Text = @"Current updated Time: " + DateTime.Now.ToShortTimeString();
        return false;
    }

In this there is already registered student with matching result but still I am getting count=0. Don't know how.

Requeting your suggestions.

Answer 1

First, you are open for sql-injection, use sql-parameters instead of string concatenation.

According to your actual issue, i must admit that i'm not sure what causes the problem. But using parameters with the correct types often solves such issues:

private bool Validate(string studentId, string time)
{
    DateTime registrationDate;
    if (!DateTime.TryParse(time.Trim(), out registrationDate))
        throw new ArgumentException("Time must be convertible to datetime", "time");
    int id;
    if (!int.TryParse(studentId.Trim(), out id))
        throw new ArgumentException("StudentId must be convertible to Integer", "studentId");
    string sql = @"
        SELECT * FROM tblStudent 
        WHERE   Registeration_Date=?Registeration_Date 
        AND     StudentID=?StudentID 
        ORDER BY date_time desc 
        limit 1";
    using (var msCon = new MySqlConnection("connection-string"))
    using (var da = new MySqlDataAdapter(sql, msCon))
    {
        da.SelectCommand.Parameters.AddWithValue("?Registeration_Date", registrationDate);
        da.SelectCommand.Parameters.AddWithValue("?StudentID", id);

        var table = new DataTable();
        da.Fill(table);
        return table.Rows.Count = 1;
    }
}

Answer 2

I had the same problem, more info, the sql database returns 0 row(s) but da.Tables(0).Rows.Count=1.

My problem was that before this check, I used the dataset in another SQL query. REMEMBER ALWAYS TO RESET THE DATASET BEFORE NEW SQL QUERY ANSWER, or you will have problems.