选择查询两个日期之间不起作用

Question

I have this block of code in one form of mine,

using (SqlConnection conn = new SqlConnection(Properties.Settings.Default.ConString))
{
    query = "select SUM(TotalMinutes) as TotalMin, SUM(OTMinutes) as OTMin from EmpLog where EmpID = @EmpID and LogDate >= @empdate1 and LogDate <= @empdate2";
    conn.Open();
    using (SqlCommand cmd2 = new SqlCommand(query, conn))
    {
        cmd2.Parameters.AddWithValue("@empdate1", dtp_from.Value.ToString("yyyy-MM-dd"));
        cmd2.Parameters.AddWithValue("@empdate2", dtp_to.Value.ToString("yyyy-MM-dd"));
        cmd2.Parameters.Add(new SqlParameter() { ParameterName = "@EmpID", Value = listBox1.SelectedValue.ToString() });
        var da = new SqlDataAdapter(cmd2);
        DataTable dt = new DataTable();
        da.Fill(dt);             

        decimal decminutes = Convert.ToDecimal(dt.Rows[0]["TotalMin]"].ToString());
        decimal decOTminutes = Convert.ToDecimal(dt.Rows[0]["OTMin"].ToString());

I have made a similar code work on different forms but somehow this won't work. I've tried SqlDataReader but it won't even go inside a while reader.read loop.

also upon trying out the query via ssms, it works as expected. but if i try other methods such as the sqldatareader, it goes into null exception.

EDIT: I've tried removing the WHERE clause from my query, and i'm able to return the rows.

for some odd reason, the datetimepicker values isn't properly being matched with the values in the database. even if when I casted it to string, it exactly matches, but then again, its still returning dbnull.

Answer 1

The BETWEEN Keyword

You may want to consider using the BETWEEN keyword within SQL for checking if a value exists in a given range as demonstrated in this SQL example :

SELECT SUM(TotalMinutes) AS TotalMin, 
       SUM(OTMinutes) AS OTMin 
  FROM EmpLog 
 WHERE EmpID = @EmpID 
   AND LogDate BETWEEN @empdate1 AND @empdate2"

Avoid Unnecessary Casting

Additionally, you may want to avoid explicitly casting your DateTime values as strings. The SQL provider should have no trouble translating those values into their appropriate SQL equivalents.

So applying both those changes, should give you something like this :

using (var conn = new SqlConnection(Properties.Settings.Default.ConString))
{
     conn.Open();
     var query = "SELECT SUM(TotalMinutes) AS TotalMin, SUM(OTMinutes) AS OTMin FROM EmpLog WHERE EmpID = @EmpID AND LogDate BETWEEN @empdate1 AND @empdate2";
     using (var cmd = new SqlCommand(query, conn))
     {
           cmd.Parameters.AddWithValue("@empdate1", dtp_from.Value);
           cmd.Parameters.AddWithValue("@empdate2", dtp_to.Value);
           cmd.Parameters.Add("@EmpID", listBox1.SelectedValue);
           var da = new SqlDataAdapter(cmd2);
           DataTable dt = new DataTable();
           da.Fill(dt);     

           // Other code omitted for brevity
     }        
}

Example Using DataReader

using (var conn = new SqlConnection(Properties.Settings.Default.ConString))
{
     conn.Open();
     var query = "SELECT SUM(TotalMinutes) AS TotalMin, SUM(OTMinutes) AS OTMin FROM EmpLog WHERE EmpID = @EmpID AND LogDate BETWEEN @empdate1 AND @empdate2";
     using (var cmd = new SqlCommand(query, conn))
     {
           cmd.Parameters.AddWithValue("@empdate1", dtp_from.Value);
           cmd.Parameters.AddWithValue("@empdate2", dtp_to.Value);
           cmd.Parameters.Add("@EmpID", listBox1.SelectedValue);
           using(var reader = cmd.ExecuteReader())
           {
                while(reader.Read())
                {
                     var minutes = Convert.ToDecimal(reader["TotalMin"]);
                     var otMinutes = Convert.ToDecimal(reader["OTMin"]);

                     // Do something here
                }
           }
     }        
}