In this article, I will explain you how we can bind multiple dependent DropDownList in asp.net from DataBase.

First, we need to create 3 different tables: tblCountry, tblState , tblCity. And insert related values into tables.

create table tblCountry
(
CountryId int ,
CountryName varchar(20)
)
insert into tblCountry values(1,    class='string'>'India')
insert into tblCountry values(2,    class='string'>'USA')

create table tblState
(
StateId int,
StateName varchar(20),
CountryId int
)
insert into tblState values(1,    class='string'>'Maharashtra',1)
insert into tblState values(2,    class='string'>'Gujarat',1)
insert into tblState values(3,    class='string'>'NewYork',2)
insert into tblState values(4,    class='string'>'California',2)

create table tblCity
(
 CityId int,
CityName varchar(20),
 StateId int,
)
insert into tblCity values(1,    class='string'>'Mumbai',1)
insert into tblCity values(2,    class='string'>'Pune',1)
insert into tblCity values(3,    class='string'>'Ahmedabad',2)
insert into tblCity values(4,    class='string'>'Surat',2)
insert into tblCity values(5,    class='string'>'New York City',3)
insert into tblCity values(6,    class='string'>'Rochester',3)
insert into tblCity values(7,    class='string'>'Los Angeles',4)
insert into tblCity values(8,    class='string'>'San Diego',4)

Write your connection string in web.config within tag.

add name="MyConnString" connectionString=    class='string'>"server=XXXX;database=..;uid=..;password=..;" 
    providerName="System.Data.SqlClient"/>

In designer file, create 3 DropDownList control named as ddlCountry , ddlState, ddlCity. On select of country DropDown, state related to that country will get bind. On select of state DropDown, city related to that state will get bind. And there is a clear Button to clear the state and city Dropdowns.

 DropDownList ID="ddlCountry" runat="server" OnSelectedIndexChanged="ddlCountry_SelectedIndexChanged"
 AutoPostBack="true">DropDownList>
    
 DropDownList ID="ddlState" runat="server" OnSelectedIndexChanged="ddlState_SelectedIndexChanged" 
 AutoPostBack="true">DropDownList>
    
 DropDownList ID="ddlCity" runat="server">DropDownList>
    
 Button id="btnClear" runat="server" Text="Clear" OnClick="btnClear_Click">Button>

Include the below Namespaces.

using System.Configuration;
using System.Data;
using System.Data.SqlClient;

Complete Code

string MyConnString = ConfigurationManager.ConnectionStrings[            class='string'>"MyConnString"].ConnectionString;
//retrieving connection string from web.config file
SqlConnection con = null;
SqlCommand cmd = null;

protected void Page_Load(            class='kwrd'>object sender, EventArgs e)
        {
if (!IsPostBack)
            {
                BindCountry();
            }

        }

private void BindCountry()
        {
            con = new SqlConnection(MyConnString);
            cmd = new SqlCommand(                class='string'>"select CountryId,CountryName from tblCountry", con);
            cmd.CommandType = CommandType.                class='type'>Text;
            con.Open();
SqlDataAdapter sda = new                 class='type'>SqlDataAdapter(cmd);
DataSet ds = new DataSet();
            sda.Fill(ds);
            ddlCountry.DataSource = ds;
            ddlCountry.DataTextField = "CountryName";
            ddlCountry.DataValueField = "CountryId";
            ddlCountry.DataBind();
            ddlCountry.Items.Insert(0, new ListItem(                class='string'>"--select--"));
        }

protected void ddlCountry_SelectedIndexChanged(            class='kwrd'>object sender, EventArgs e)
        {
            ddlState.Items.Clear();
            ddlCity.Items.Clear();
if (ddlCountry.SelectedIndex != 0)
            {
                con = new SqlConnection(MyConnString);
                cmd = new SqlCommand(                    class='string'>"select StateId,StateName from tblState where CountryId=" 
                    + ddlCountry.SelectedItem.Value +                         class='string'>"", con);
                cmd.CommandType = CommandType.                    class='type'>Text;
                con.Open();
SqlDataAdapter sda = new                     class='type'>SqlDataAdapter(cmd);
DataSet ds = new DataSet();
                sda.Fill(ds);
                ddlState.DataSource = ds;
                ddlState.DataTextField = "StateName";
                ddlState.DataValueField = "StateId";
                ddlState.DataBind();
                ddlState.Items.Insert(0, new ListItem(                    class='string'>"--select--"));
            }
        }

protected void ddlState_SelectedIndexChanged(            class='kwrd'>object sender, EventArgs e)
        {
            ddlCity.Items.Clear();
if (ddlState.SelectedIndex != 0)
             {
                con = new SqlConnection(MyConnString);
                 cmd = new SqlCommand(                     class='string'>"select CityId,CityName from tblCity where StateId="
                      + ddlState.SelectedItem.Value +                          class='string'>"", con);
                 cmd.CommandType = CommandType.                     class='type'>Text;
                 con.Open();
SqlDataAdapter sda = new                      class='type'>SqlDataAdapter(cmd);
DataSet ds = new DataSet();
                 sda.Fill(ds);
                 ddlCity.DataSource = ds;
                 ddlCity.DataTextField = "CityName";
                 ddlCity.DataValueField = "CityId";
                 ddlCity.DataBind();
                 ddlCity.Items.Insert(0, new ListItem(                     class='string'>"--select--"));
             }

        }

protected void btnClear_Click(            class='kwrd'>object sender, EventArgs e)
        {
            ddlState.Items.Clear();
            ddlCity.Items.Clear();
            ddlCountry.SelectedIndex = 0;

        }