1. limit 记录个数
 getData() {
        db.collection("demoUser").limit(3).get()
        .then(res=>{
            console.log(res)
        })
    },
2. orderBy 排序
    getData() {
        db.collection("demoUser").limit(3).orderBy("time", "desc").get() // asc 正序，desc 逆序
        .then(res=>{
            console.log(res)
        })
    },

3. skip .skip(n) // 过滤n条数据
       getData() {
        db.collection("demoUser").limit(3).orderBy("time", "desc").skip(2).get()
        .then(res=>{
            console.log(res)
        })
    },

4. field 选择查询的属性
    getData() {
        db.collection("demoUser").limit(3).orderBy("time", "desc").skip(1).field({
            title:true,
            author:true
        }).get()
        .then(res=>{
            console.log(res)
        })
    },