router.get("/goods/:goodsid",function (req,res,next) {        var sql="select * from table where ";        if(req.query.a!=null) {
            console.log(req.query.limit);
            sql+=" a="+req.query.a;
        }
        if(req.query.b!=null){
            sql+=" and b="+req.query.b;
        }        if(req.query.c!=null){
            sql+=" and c="+req.query.c;
        }
        //...    })sql构造这样很繁琐,是否有更有效的方法