CRUD Operations using 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.SqlClient;

namespace CsharpCorner
{
    public partial class db : System.Web.UI.Page
    {
        SqlConnection cn;
        SqlCommand comm;
        SqlDataReader dr;
        string st, cnst;

        protected void btnselect_Click(object sender, EventArgs e)
        {
   cnst = @"Data Source=(LocalDB\v11.0;AttachDbFilename=C:\Users\enduser\Documents\student.mdf;Integrated Security=True;Connect Timeout=30";
            cn = new SqlConnection(cnst);
            cn.Open();
            st = "select * from emp where empno=" + TextBox1.Text;
            comm = new SqlCommand(st, cn);
            dr=comm.ExecuteReader();

            if(dr.Read()==true)
            {
                TextBox2.Text = dr.GetString(1);
                DropDownList1.SelectedValue = dr.GetString(2);

            }
            else
            {
                Response.Write("no such record exist");
            }
            dr.Close();
            cn.Close();
        }

        protected void btnInsert_Click(object sender, EventArgs e)
        {
             cnst = @"Data Source=(LocalDB)\v11.0;AttachDbFilename=C:\Users\enduser\Documents\student.mdf;Integrated Security=True;Connect Timeout=30";
            cn = new SqlConnection(cnst);
            cn.Open();
            //insert into emp values(1,'kiran','cse')
            st = "insert into emp values("+TextBox1.Text+",'"+TextBox2.Text+"','"+DropDownList1.SelectedValue+"')";
            comm = new SqlCommand(st, cn);
            comm.ExecuteNonQuery();
            cn.Close();
            Response.Write("inserted record");
            Clear();

        }

        protected void btnupdate_Click(object sender, EventArgs e)
        {
            cnst = @"Data Source=(LocalDB)\v11.0;AttachDbFilename=C:\Users\enduser\Documents\student.mdf;Integrated Security=True;Connect Timeout=30";
            cn = new SqlConnection(cnst);
            cn.Open();
            //UPDATE table_name SET column1=value1,column2=value2 WHERE some_column=some_value;

            st = "update emp set ename='" + TextBox2.Text + "',dept='" + DropDownList1.SelectedValue + "' where empno=" +TextBox1.Text;
            comm = new SqlCommand(st, cn);
            comm.ExecuteNonQuery();
            cn.Close();
            Response.Write("updated record");
            Clear();
        }

        protected void btndelete_Click(object sender, EventArgs e)
        {
            cnst = @"Data Source=(LocalDB)\v11.0;AttachDbFilename=C:\Users\enduser\Documents\student.mdf;Integrated Security=True;Connect Timeout=30";
            cn = new SqlConnection(cnst);
            cn.Open();
            //DELETE FROM table_name WHERE some_column = some_value;
            st = "delete from emp where empno="+TextBox1.Text;
            comm = new SqlCommand(st, cn);
            comm.ExecuteNonQuery();
            cn.Close();
            Response.Write("deleted record");
            Clear();

        }

        protected void btnclear_Click(object sender, EventArgs e)
        {
            Clear();
        }

        private void Clear()
        {
            TextBox1.Text = "";

            TextBox2.Text = "";

            TextBox1.Focus();
        }


    }
}