<%@ Import Namespace=”System.Data” %>
<%@ Import Namespace=”System.Data.Odbc” %> <script
language=”C#” runat=”server”>
    void Application_Start()
    {
        //Connection string for SQL Server with trusted
connection
        Application.Add (”DB_CONNECTION_STRING”,”DRIVER={SQL
Server};SERVER=ServerName;DATABASE=DataBaseName;Trusted_connection=yes;”);
    }
    void Session_Start()
    {
        //open connection to database
        OdbcConnection  cnConnection = new OdbcConnection
(Application.Get(”DB_CONNECTION_STRING”).ToString());
        cnConnection.Open();
        Session.Add (”DB_CONNECTION”, cnConnection );
    }
    void Session_End()
    {
        //a graceful disconnect
        OdbcConnection cnConnection = (OdbcConnection)
Session["DB_CONNECTION"];
        cnConnection.Close ();
    }
</script>

1)We can't add connection string in global.asax file. we can
create connection string in web.config file only.
2)global.asax,web.config,Default.aspx files and
app_code,app_data,app_themes folders.

Connection String can be stored as an Application Variable.

it is possible only by using session but it is not 
good practics

no,we can't add the connection string in global.asax..
global.asax is a text file,It is use to define the global 
variable.