Prevent double booking ASP.NET C# - asp.net

Question

I 'am trying to create a vacation planner in asp.net with a SQL Server Express database.
When you add a vacation, you have to add a begintime, endtime and of course who you are.
Is there a way to show an error when there is already vacation with the same time and the same employee as the one they try to add.
This way the system will prevent double vacations. Otherwise Gridview table shows two the same name in two rows with the same days. I want to prevent people from double booking.
Let's say Person A has already booked StartDate = 2016-03-06 and EndDate = 2016-03-10. Than means I want to prevent someone else to book the same StartDate, EndDate, between start and end date, not even earlier date and date between start and end date, I mean for example between 2016-03-01 and 2016-03-06 , 2016-03-07, 2016-03-08... because those dates are between start and end dates.
I tried to create a method like following and then I don't know what to do. I would appreciate for any help
// Noe edited to insert button instead method:
protected void btnInsertVacation_Click(object sender, EventArgs e)
{
string cs = ConfigurationManager.ConnectionStrings["ResursplaneringConnectionString"].ConnectionString;
TextDateTime.Text = Calendar1.SelectedDate.ToShortDateString();
using (SqlConnection con = new SqlConnection(cs))
{
string check = "SELECT EmployeeId, StartDate, EndDate FROM vacation WHERE (EmployeeId = #EmployeeId) AND(StartDate <= #NewEndDate) AND(EndDate >= #NewStartDate)";
SqlCommand cmd = new SqlCommand(check, con);
cmd.Parameters.AddWithValue("#EmployeeId", DropDownEmployee.SelectedValue);
cmd.Parameters.AddWithValue("#NewEndDate", txtStartDate.Text);
cmd.Parameters.AddWithValue("#NewStartDate", txtEnd.Text);
con.Open();
using (SqlDataReader rdr = cmd.ExecuteReader())
{
while (rdr.Read())
{
if (rdr.HasRows)
{
Response.Write("Dubbel booking");
}
else
{
string insertVacation = "INSERT INTO Planering (StartDate, EndDate, EmployeeId ) VALUES (#StartDate, #EndDate, #EmployeeId)";
SqlCommand cmd2 = new SqlCommand(insertVacation, con);
SqlParameter paramPlaneringStart = new SqlParameter("#StartDate", txtStartDate.Text);
cmd2.Parameters.Add(paramPlaneringStart);
SqlParameter paramPlaneringEnd = new SqlParameter("#EndDate", txtEnd.Text);
cmd2.Parameters.Add(paramPlaneringEnd);
SqlParameter paramEmployeeId = new SqlParameter("#EmployeeId", DropDownEmployee.SelectedValue);
cmd2.Parameters.Add(paramEmployeeId);
con.Open();
cmd2.ExecuteNonQuery();
}
}
}
}
}

Answer 1

For some clarity, let's name the arguments of your method NewStartDate and NewEndDate, to avoid confusion with StartDate and EndDate we'll use for the columns in the database table.
What you want to do in your SQL query is:
SELECT EmployeeId, StartDate, EndDate FROM vacation
WHERE
(EmployeeId = #EmployeeId)
AND (StartDate <= #NewEndDate) AND (EndDate >= #NewStartDate)
Try to draw possible overlapping as segments on a piece of paper, and you should see this will do it.
If this returns any rows, do not allow to create the new booking.