以下sql插入到表空值,结果是null字符串
String sql = "update orderdetail set isapproveddate='"+ null + "' where id='" + orderdetailid + "'";
String sql = "update orderdetail set isapproveddate='"+ null + "' where id='" + orderdetailid + "'";
改成这样试试
String sql = "update orderdetail set isapproveddate=null where id='" + orderdetailid + "'";