Dropdownlist for selected department is not filtering

<pre>private void BindGrid()
        {
            DataTable dt = new DataTable();
            
            string query = " select * from gvdetails17 WHERE 1=1 ";
            SqlCommand cmd = new SqlCommand();
            cmd.Connection = cn;


            if (ddlAddSalary1.SelectedValue != "" && ddlAddSalary1.SelectedValue != "--Select--")
            {
               
                string[] sal = ddlAddSalary1.SelectedValue.Split('-');
                string from = sal[0];
                string to = sal[1];


                query += " and Sal between  @fromsal and @tosal ";
                cmd.Parameters.AddWithValue("@fromsal", from);
                cmd.Parameters.AddWithValue("@tosal", to);
                
            }
            if (ddlAge.SelectedValue != "" && ddlAge.SelectedValue != "--Select--")
          
            {
               
                string[] age = ddlAge.SelectedValue.Split('-');
                string from1 = age[0];
                string to1 = age[1];

                query += " and Age between  @fromage and @toage ";
                cmd.Parameters.AddWithValue("@fromage", from1);
                cmd.Parameters.AddWithValue("@toage", to1);
                 
            }


            if (ddlDepartment.SelectedValue == "")
            {
                cmd.Parameters.AddWithValue("@DepartmentName", ddlDepartment.SelectedValue);
            }
            cmd.CommandText = query;
            cmd.CommandType = CommandType.Text;
            SqlDataAdapter da = new SqlDataAdapter(cmd);
            da.Fill(dt);
            gvDetails.DataSource = dt;
            gvDetails.DataBind();
        }

        protected void txt_SearchName_TextChanged1(object sender, EventArgs e)
        {
            cn.Open();
            SqlCommand cmd = new SqlCommand("select * from gvdetails17 where EmpName LIKE '% " + txt_SearchName.Text + "%'", cn);
            DataTable dt = new DataTable();
            SqlDataAdapter da = new SqlDataAdapter(cmd);
            da.Fill(dt);
            gvDetails.DataSource = dt;
            cn.Close();
        }

protected void ddlDepartment_SelectedIndexChanged(object sender, EventArgs e)
        {

            BindGrid();

        }