protected void Page_Load(object sender, EventArgs e)
    {
        if (!IsPostBack)
        {
            DataTable dt;
            String SQL= "SELECT B.Name [Brand Name],C.Name [Category Name], " +
                    "P.Name [Product Name] FROM " +
                    "Brand B, Category C, Product P " +
                    "WHERE B.ID=P.BrandID AND C.ID=P.CategoryID Order BY 1,2,3";
            

            string sConstr = ConfigurationManager.ConnectionStrings["TestConnection"].ConnectionString;
            using (SqlConnection conn = new SqlConnection(sConstr))
            {
                using (SqlCommand comm = new SqlCommand(SQL, conn))
                {
                    conn.Open();
                    using (SqlDataAdapter da = new SqlDataAdapter(comm))
                    {
                        dt = new DataTable("tbl");
                        da.Fill(dt);
                    }
                }
            }
            
            GridView1.DataSource = dt;
            GridView1.DataBind();

        }

The best practice is to wrap up all code under using statement. 

If you look at the code you will find that i have wrapped up a

ll code under connection object as well as sql command. Keep

 in mind that when corresponding "using" statement reached 

at the end then asp.net automatically clear all variables imm

ediately within the scope. You do not need to dispose those 

manually. Such as here i don't close the connection, sqlcommand.

 For ease understanding here i am using datatable. You can use

 any ado.net component whichever you like. But keep in mind to

 wrap up the connection object within "Using" statement.