Search checkbox values in gridview

Hi,

I have a dropdown list with checkbox in the page and when I select multiple options and search its not returning all the selected values in the grid. It returns only one selected value. Here is the code I use it for search. Any suggestions or change in the code. I have binded one gridview column to that checkbox. Checkbox is present outside the gridview. Any suggestions or change in the code?




Below is the code on button search I am trying to fetch the checkbox values
  protected void btnSearchGroup_Click(object sender, EventArgs e)
        {
            string selectedValues = string.Empty;

            foreach (ListItem item in cblGroup.Items)
            {
                if (item.Selected)
                    selectedValues += item.Value+",";
            }

            if (selectedValues != string.Empty)
                selectedValues = selectedValues.Remove(selectedValues.Length - 1);//To remove the last comma;


            SqlConnection con = new SqlConnection(strcon);
            SqlCommand cmd = new SqlCommand("select * from AppInvent_Test where Designation in ('" + selectedValues + "')", con);
            SqlDataAdapter Adpt = new SqlDataAdapter(cmd);
            DataTable dt = new DataTable();
            Adpt.Fill(dt);
            GridView1.DataSource = dt;
            GridView1.DataBind();

        }