-- set as optional parameter
@area int = -1

-- area = -1 means true
WHERE
    ( area = -1
        OR
      area = @area
    )

        /// <summary>
        /// 初始化下拉框控件
        /// </summary>
        /// <param name="ddlA"></param>
        public void InitializeDropDownList(DropDownList ddlA)
        {
            ddlA.Items.Clear();
            ddlA.Items.Add( new ListItem("不限", "0"));
            ddlA.Items.Add( new ListItem("50-100", "1"));
            ddlA.Items.Add( new ListItem("101-200", "2"));
            ddlA.Items.Add( new ListItem("201-300", "3"));
            ddlA.Items.Add( new ListItem("300+", "4"));

        }


        /// <summary>
        /// 根据下拉框控件的选中项获取sql查询语句
        /// </summary>
        /// <param name="ddlA"></param>
        /// <returns></returns>
        public string GetSelectSql(DropDownList ddlA)
        {
            int selectedValue = 0;
            string strSql = string.Empty;
            try
            {
                selectedValue = int.Parse(ddlA.SelectedValue);
            }
            catch
            {
            }

            switch(selectedValue)
            {
                case 1:
                    // 在此处编写返回的Sql语句
                    break;
                case 2:
                    break;
                case 3:
                    break;
                case 4:
                    break;

                    
                case 0:
                default:
                    break;
            }

            return strSql;
        }