Implement level-2 linkage menu C # with complete database SQL

---------------- Db. cs ------------------------------

// TODO: add the constructor logic here
//
}
Public static SqlConnection createConnection ()
{
SqlConnection con = new SqlConnection ("server =.; database = list; uid = sa; pwd = ;");
Return con;
}
}
}


--------------------------. Cs source file --------------------------------

Private void Page_Load (object sender, System. EventArgs e)
{
If (! Page. IsPostBack)
{
SqlConnection con = db. createConnection ();
Con. Open ();
SqlCommand cmd = new SqlCommand ("select * from sheng", con );
SqlDataReader sdr = cmd. ExecuteReader ();
This. sheng. DataSource = sdr;
This. sheng. DataTextField = "sheng ";
This. sheng. DataValueField = "shengid ";
This. sheng. DataBind ();
Sdr. Close ();
Cmd. CommandText = "select * from city where shengid = 1 ";
Sdr = cmd. ExecuteReader ();
This. city. DataSource = sdr;
This. city. DataTextField = "cityname ";
This. city. DataValueField = "cityid ";
This. city. DataBind ();
Sdr. Close ();
Con. Close ();
}
}
# Code generated by region Web Form Designer
Override protected void OnInit (EventArgs e)
{
//
// CODEGEN: This call is required by the ASP. NET Web form designer.
//
InitializeComponent ();
Base. OnInit (e );
}

/// <Summary>
/// The designer supports the required methods-do not use the code editor to modify
/// Content of this method.
/// </Summary>
Private void InitializeComponent ()
{
This. sheng. SelectedIndexChanged + = new System. EventHandler (this. DropDownList1_SelectedIndexChanged );
This. Button1.Click + = new System. EventHandler (this. button#click );
This. Calendar1.SelectionChanged + = new System. EventHandler (this. calendar#selectionchanged );
This. rdl. SelectedIndexChanged + = new System. EventHandler (this. drl_SelectedIndexChanged );
This. mar. SelectedIndexChanged + = new System. EventHandler (this. mar_SelectedIndexChanged );
This. Load + = new System. EventHandler (this. Page_Load );

}
# Endregion

Private void button#click (object sender, System. EventArgs e)
{

}
Private void dropdownlistpolicselectedindexchanged (object sender, System. EventArgs e)
{
SqlConnection con = db. createConnection ();
Con. Open ();
SqlCommand cmd = new SqlCommand ("select * from city where shengid =" + this. sheng. SelectedValue. ToString (), con );
SqlDataReader sdr = cmd. ExecuteReader ();
This. city. DataSource = sdr;
This. city. DataTextField = "cityname ";
This. city. DataValueField = "cityid ";
This. city. DataBind ();
Sdr. Close ();
Con. Close ();
}

----------------------- SQL file -----------------------

Create database list

Use list

Create table sheng
(
Shengid int primary key,
Sheng varchar (50) not null,
)

Insert into sheng values (1, 'sichuan ')
Insert into sheng values (2, 'anhui ')
Insert into sheng values (3, 'henan ')
Insert into sheng values (4, 'chongqing ')
Insert into sheng values (5, 'guangxi ')

Select * from sheng

Create table city
(
Cityid int primary key,
Shengid int foreign key references sheng (shengid ),
Cityname varchar (50) not null,
)

Insert into city values (1, 1, 'chengdu ')
Insert into city values (2, 1, 'mianyang ')
Insert into city values (3, 1, 'guangyuan ')
Insert into city values (4, 2, 'hefei ')
Insert into city values (5, 2, 'bengbu ')
Insert into city values (6, 2, 'huainan ')
Insert into city values (7,3, 'zhengzhou ')
Insert into city values (8, 3, 'kaifeng ')
Insert into city values (10, 3, 'shangqiu ')
Insert into city values (11,4, 'dianjiang ')
Insert into city values (12, 4, 'jiangjin ')
Insert into city values (13, 4, 'chongqing ')
Insert into city values (14,5, 'nanning ')
Insert into city values (15,5, 'liuzhou ')
Insert into city values (16, 5, 'White ')

Select * from city

--------- If a friend learns to use it, it is better to see clearly. I write while doing it, but it is not a successful execution ------