举个例子呗
下拉框参数为空选择全部-https://help.finereport.com/doc-view-2394.html
${IF(len(开始日期) == 0,""," AND DATE_FORMAT(日期,'%Y-%m-%d') >= ('"+开始日期+"')")}
${IF(len(结束日期) == 0,""," AND DATE_FORMAT(日期,'%Y-%m-%d') <= ('"+结束日期+"')")}