Problem view records in gridview in asp.net

hi friends,
i want to check from label1(last_datetime),only view, after the records(last_datetime) in gridview.
>(greater than) is not displayed the records)

string CommandText = "select * from data where cdate > '" + Convert.ToDateTime(last_datetime) + "' Order By cdate desc limit 20";

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public void filldate()
{

cmd.CommandText = "select top 1 cdate from data";
Label1.Text = Convert.ToString(cmd.ExecuteScalar());


}

}
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public void fillgrid()
{

string last_datetime = Label1.Text;

string ConnectionString = @" ";

string CommandText = "select * from data where cdate > '" + Convert.ToDateTime(last_datetime) + "' Order By cdate desc limit 20";

}

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and


how to if label1.text display no records if not in database.

please give the correct solution,

thanks in advance.