How can I select a value from table and save it into a variable?

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C#

ASP.NET

Hello guys,

please I need your help. I have a code which get max value from Request table and save the returned value into a variable. then use this variable with insert statement to save into Vacation table to use it when I want display the requests. Here is my code

<pre lang="c#">

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.Data;
using System.Data.SqlClient;
using System.Configuration;

public partial class Vavation : System.Web.UI.Page
{
Dbclass db = new Dbclass();
string s = "";
string d = "";
int R;
string firstVariable = string.Empty;
protected void Page_Load(object sender, EventArgs e)
{
try
{
if(!IsPostBack)
{
tbid.Text = Session["uid"].ToString();
lblDept.Text = Session["Did"].ToString();
lblid.Text = tbid.Text;
string s = "Select * From Employee where National_ID='" + lblid.Text + "'";
DataTable dt = db.getTable(s);
if (dt != null && dt.Rows.Count > 0)
{
tbid.Text = dt.Rows[0]["National_ID"].ToString();
tbname.Text = dt.Rows[0]["FirstName"].ToString() + ' ' + dt.Rows[0]["LastName"].ToString();
lblDept.Text = dt.Rows[0]["Dept_ID"].ToString();
}
string d = "SELECT Balance FROM Vacation WHERE (National_ID = '" + lblid.Text + "') AND (StartDate =(SELECT MAX(StartDate) FROM Vacation WHERE (National_ID ='" + lblid.Text + "'))) AND (Balance = (SELECT MIN(Balance) FROM Vacation WHERE (National_ID ='" + lblid.Text + "')))";
DataTable t = db.getTable(d);
if (t != null && t.Rows.Count > 0)
{
lbldays.Text = t.Rows[0]["Balance"].ToString();
}
}

}
catch(Exception ex)
{
lberr.Text = ex.Message;
}
}

protected void btnadd_Click(object sender, EventArgs e)
{
AddVaction();
GetMax();
int Remaine = Convert.ToInt32(lbldays.Text);
DateTime start = DateTime.Parse(tbstartvaca.Text).Date;
if (Remaine <= 30 && Remaine >= 0 && start >= DateTime.Now)
{
TimeSpan remaindate;
DateTime end = DateTime.Parse(tbendvaca.Text).Date;
TimeSpan vacation = TimeSpan.Parse(lbldays.Text);
int Balance = (int)vacation.TotalDays;
TimeSpan total;
if (start > end)
{
lberr.Text = "Please check again on Starting date";
return;
}

remaindate = end - start;
int days = (int)remaindate.TotalDays;
lberr.Text = "you have left with " + remaindate.TotalDays + "days.";
total = vacation - remaindate;
int T = (int)total.TotalDays;
lbldays.Text = "you have left with " + total.TotalDays + "days.";

s = "Insert into Vacation (Balance, National_ID, NoOfDays, StartDate, EndDate, Dept_ID, Req_date, Req_ID) values ('" + T + "', '" + lblid.Text + "', '" + days + "', '" + tbstartvaca.Text + "', '" + tbendvaca.Text + "', '" + lblDept.Text + "', '" + DateTime.Now + "', '" + R + "')";
db.Run(s);

}
else
{
lberr.Text = "You Don't a Balance of Vacation OR Start of Vacation is less than date of day ";
}

}

private void GetMax()
{
d = "select MAX(Req_ID) from Request";
lblReq.Text = d;
DataTable dt = db.getTable(d);
if (dt != null && dt.Rows.Count > 0)
{
R = Convert.ToInt32(dt.Rows[0]["Req_ID"].ToString());

}

}

private void AddVaction()
{
s = "Insert into Request (National_ID, Type_Req, Dept_ID,Date) values ('" + lblid.Text + "', 'Vacation','" + lblDept.Text + "', '" + DateTime.Now + "')";
db.Run(s);

}
}

when I run the code I get Max Req_ID "zero" although the code running with out errors and the Max Req_ID should be the latest value.

please anyone has idea to solve the problem help me. I do not where the problem is. is it in database structure or in the code.

What I have tried:

I tried many code but I could not get any value.