How to filter data using multiple CheckBoxes from CheckBoxList in GridView in ASP.Net

Check this example

 <asp:CheckBoxList ID="chkProductTypes" runat="server" AutoPostBack="true" OnSelectedIndexChanged="ProductsType_Changed">
    </asp:CheckBoxList>
    <br />
    <asp:GridView ID = "gvProducts" runat="server" AutoGenerateColumns="false">
        <Columns>
            <asp:BoundField DataField="ProductId" HeaderText="Product Id" />
            <asp:BoundField DataField="Name" HeaderText="Name" />
        </Columns>
    </asp:GridView>

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

    protected void Page_Load(object sender, EventArgs e)
    {
        if (!this.IsPostBack)
        {
            string query = "SELECT 'ALL (' + CAST((SELECT COUNT(ProductId) FROM Products) AS VARCHAR(10)) + ')' [Type], 0 [TypeId]";
            query += " UNION ALL ";
            query += "SELECT [Type] + ' (' + CAST((SELECT COUNT(ProductId) FROM Products WHERE ProductTypeId = TypeId) AS VARCHAR(10)) + ')' [Type], TypeId FROM ProductTypes";
            chkProductTypes.DataSource = this.GetData(query);
            chkProductTypes.DataTextField = "Type";
            chkProductTypes.DataValueField = "TypeId";
            chkProductTypes.DataBind();
            chkProductTypes.Items[0].Selected = true;
            ProductsType_Changed(null, EventArgs.Empty);
        }
    }

    private DataTable GetData(string query)
    {
        string conString = ConfigurationManager.ConnectionStrings["constring"].ConnectionString;
        SqlCommand cmd = new SqlCommand(query);
        using (SqlConnection con = new SqlConnection(conString))
        {
            using (SqlDataAdapter sda = new SqlDataAdapter())
            {
                cmd.Connection = con;
                sda.SelectCommand = cmd;
                using (DataTable dt = new DataTable())
                {
                    sda.Fill(dt);
                    return dt;
                }
            }
        }
    }

    protected void ProductsType_Changed(object sender, EventArgs e)
    {
        string query = "SELECT * FROM Products";
        if (chkProductTypes.Items[0].Selected)
        {
            gvProducts.DataSource = this.GetData(query);
            gvProducts.DataBind();
        }
        else
        {
            bool isSelected = false;
            query += " WHERE ProductTypeId IN (";
            foreach (ListItem item in chkProductTypes.Items)
            {
                if (item.Selected)
                {
                    isSelected = true;
                    query += item.Value + ",";
                }
            }
            query = query.Substring(0, query.Length - 1);
            query += ")";
            if (isSelected)
            {
                gvProducts.DataSource = this.GetData(query);
            }
            else
            {
                gvProducts.DataSource = null;
            }
            gvProducts.DataBind();
        }
    }