C＃：SQL“选择顶部。”查询不返回任何行？

Question

I need to:

1) Filter Rows by "NodeParent" (which I provide)

2) Sort rows by "Time" and get the most recent entry

Here's what I wrote:

using (SqlConnection con = new SqlConnection(conString))
{
        SqlCommand cmd = new SqlCommand("SELECT TOP 1 NodeID FROM ActivityTable WHERE NodeParent='" + nodeid_previous + "'" + " ORDER BY Time DESC", con);

        con.Open();
        SqlDataReader rdr = cmd.ExecuteReader();
        if (rdr.HasRows)
        {
             while (rdr.Read())
             {
                     nodeid_previous = rdr["NodeID"].ToString();
                     break;
             }
             rdr.Close();
        }
        else
        {
             //so on and so forth..
        }
}

This however doesn't return any results even though I HAVE rows which abide by these conditions. Is the query correct folks? :)

Answer 1

Before

    SqlCommand cmd = new SqlCommand("SELECT TOP 1 NodeID FROM ActivityTable WHERE NodeParent='" + nodeid_previous + "'" + " ORDER BY Time DESC", con);

After - Limit is added to the end of your query, rather than in the beginning of you select list like MS SQL Server's Top command

    SqlCommand cmd = new SqlCommand("SELECT NodeID FROM ActivityTable WHERE NodeParent='" + nodeid_previous + "'" + " ORDER BY Time DESC Limit 1", con);

Answer 2

You have to use "Limit 1" in the end. There is no "TOP 1" in MYSQL. I guess it is SQLServer command.

Do:

SqlCommand cmd = new SqlCommand("SELECT NodeID FROM ActivityTable WHERE NodeParent='" + nodeid_previous + "'" + " ORDER BY Time DESC Limit 1", con);