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MongoDB
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romashi  >《前端》

2020.09.03

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任何数据库中查询都是最麻烦的，在MongoDB中对于查询的支持非常到位，有关系运算，逻辑运算，数组运算等等
首先对于数据的操作核心语法：”db.集合名称.find({查询条件} , {设置显示的字段})“

  • 查询一条文档：（查询一条表中数据）：

db.dept.findOne()
输出：{
"_id" : ObjectId("5be8eab516d1fcb72bc0dd34"),
"deptno" : 10,
"dname" : "xc",
"loc" : "北京"
}
db.infos.find({'url':'www.hh.cn'})
输出：{ "_id" : ObjectId("5be917a716d1fcb72bc0dd3b"), "url" : "www.hh.cn" }

对于设置的显示字段严格来讲就称为数据的投影操作，（从一条数据的全部字段中拿出部分想要显示的字段来显示出来，就叫投影操作）如果不需要显示的字段设置”0“，而需要显示的设置为‘1’：
比如不想要显示_id：

db.infos.find({'url':'www.hh.cn'},{'_id':0})
或者：
db.infos.find({'url':'www.hh.cn'},{'_id':0,'url':1})

不部分情况下这种投影操作的意义不大。同时对于数据的查询也可以使用‘pretty()’函数进行漂亮的显示。数据列多的时候一定可以看出漂亮的显示。

  • 关系运算查询：
    MongoDB中支持关系查询操作：大于（$gt）、小于（$lt）、大于等于（$gte）、小于等于（$lte）、不等于（$ne）、等于（key:value）。
    查询name是张三的数据：

db.students.find({'name':'张三'}).pretty()

{    "_id" : ObjectId("5be9271d16d1fcb72bc0dd41"),    "name" : "张三",    "sex" : "男",    "age" : 19,    "score" : 89,    "address" : "西湖区"}

查询年龄大于19的数据：

db.students.find({'age':{'$gt':19}})
{ "_id" : ObjectId("5be9274916d1fcb72bc0dd42"), "name" : "李思", "sex" : "男", "age" : 20, "score" : 69, "address" : "拱墅区" }
{ "_id" : ObjectId("5be927af16d1fcb72bc0dd45"), "name" : "刘琦", "sex" : "女", "age" : 21, "score" : 0, "address" : "上城区" }
查询姓名不等于王五，同时年龄小于19的数据：
db.students.find({'name':{'$ne':'王五'},'age':{'$lt':19}})
{ "_id" : ObjectId("5be9278a16d1fcb72bc0dd44"), "name" : "赵柳", "sex" : "女", "age" : 17, "score" : 66, "address" : "余杭区" }

逻辑运算查询：
与（$and）、或（$or）、非（$not或者$nor）

查询年龄在19~20岁的数据：
db.students.find({'age':{'$gte':19,'$lte':20}})
{ "_id" : ObjectId("5be9271d16d1fcb72bc0dd41"), "name" : "张三", "sex" : "男", "age" : 19, "score" : 89, "address" : "西湖区" }
{ "_id" : ObjectId("5be9274916d1fcb72bc0dd42"), "name" : "李思", "sex" : "男", "age" : 20, "score" : 69, "address" : "拱墅区" }

And比较容易，只需要用逗号分隔若干个条件即可。

查询年龄大于19或者成绩大于90分的数据：
db.students.find({'$or':[{'age':{'$gt':19}},{'socre':{'$gt':90}}]})
{ "_id" : ObjectId("5be9274916d1fcb72bc0dd42"), "name" : "李思", "sex" : "男", "age" : 20, "score" : 69, "address" : "拱墅区" }
{ "_id" : ObjectId("5be927af16d1fcb72bc0dd45"), "name" : "刘琦", "sex" : "女", "age" : 21, "score" : 0, "address" : "上城区" }
上面用到了”或“，也可以进行”或“的求反：
db.students.find({'$nor':[{'age':{'$gt':19}},{'socre':{'$gt':90}}]})
{ "_id" : ObjectId("5be9271d16d1fcb72bc0dd41"), "name" : "张三", "sex" : "男", "age" : 19, "score" : 89, "address" : "西湖区" }
{ "_id" : ObjectId("5be9277416d1fcb72bc0dd43"), "name" : "王五", "sex" : "女", "age" : 18, "score" : 100, "address" : "余杭区" }
{ "_id" : ObjectId("5be9278a16d1fcb72bc0dd44"), "name" : "赵柳", "sex" : "女", "age" : 17, "score" : 66, "address" : "余杭区" }

注意，使用或的时候要用到数组结构。

求模运算：
模的运算使用”$mod“来完成，语法”{$mod:[数字，余数]}“：

db.students.find({'age':{'$mod':[20,0]}})
{ "_id" : ObjectId("5be9274916d1fcb72bc0dd42"), "name" : "李思", "sex" : "男", "age" : 20, "score" : 69, "address" : "拱墅区" }
db.students.find({'age':{'$mod':[20,1]}})
{ "_id" : ObjectId("5be927af16d1fcb72bc0dd45"), "name" : "刘琦", "sex" : "女", "age" : 21, "score" : 0, "address" : "上城区" }

范围查询：
只要是数据库必须存在”$in“（在范围之中）、”$nin“（不在范围之中）
查询姓名是张三王五李思的信息：

db.students.find({'name':{'$in':['张三','李思','王五']}})
{ "_id" : ObjectId("5be9271d16d1fcb72bc0dd41"), "name" : "张三", "sex" : "男", "age" : 19, "score" : 89, "address" : "西湖区" }
{ "_id" : ObjectId("5be9274916d1fcb72bc0dd42"), "name" : "李思", "sex" : "男", "age" : 20, "score" : 69, "address" : "拱墅区" }
{ "_id" : ObjectId("5be9277416d1fcb72bc0dd43"), "name" : "王五", "sex" : "女", "age" : 18, "score" : 100, "address" : "余杭区" }

不在上面范围之内：

db.students.find({'name':{'$nin':['张三','李思','王五']}})
{ "_id" : ObjectId("5be9278a16d1fcb72bc0dd44"), "name" : "赵柳", "sex" : "女", "age" : 17, "score" : 66, "address" : "余杭区" }
{ "_id" : ObjectId("5be927af16d1fcb72bc0dd45"), "name" : "刘琦", "sex" : "女", "age" : 21, "score" : 0, "address" : "上城区" }

范围的操作也需要用到数组结构。

数组查询：
因为MongoDB是支持数组保存的，所以也需要针对数组的查询。可以使用几个运算符：$all、$size、$slice

  • 保存一部分的数组内容：

db.students.insert({'name':'测试-A','course':['语文','数学','英语']})
WriteResult({ "nInserted" : 1 })
db.students.insert({'name':'测试-B','course':['语文','数学']})
WriteResult({ "nInserted" : 1 })
db.students.insert({'name':'测试-C','course':['语文','数学','英语','科学']})
WriteResult({ "nInserted" : 1 })
db.students.find()
{ "_id" : ObjectId("5be9271d16d1fcb72bc0dd41"), "name" : "张三", "sex" : "男", "age" : 19, "score" : 89, "address" : "西湖区" }
{ "_id" : ObjectId("5be9274916d1fcb72bc0dd42"), "name" : "李思", "sex" : "男", "age" : 20, "score" : 69, "address" : "拱墅区" }
{ "_id" : ObjectId("5be9277416d1fcb72bc0dd43"), "name" : "王五", "sex" : "女", "age" : 18, "score" : 100, "address" : "余杭区" }
{ "_id" : ObjectId("5be9278a16d1fcb72bc0dd44"), "name" : "赵柳", "sex" : "女", "age" : 17, "score" : 66, "address" : "余杭区" }
{ "_id" : ObjectId("5be927af16d1fcb72bc0dd45"), "name" : "刘琦", "sex" : "女", "age" : 21, "score" : 0, "address" : "上城区" }
{ "_id" : ObjectId("5be93fb416d1fcb72bc0dd46"), "name" : "测试-A", "course" : [ "语文", "数学", "英语" ] }
{ "_id" : ObjectId("5be93fc016d1fcb72bc0dd47"), "name" : "测试-B", "course" : [ "语文", "数学" ] }
{ "_id" : ObjectId("5be93fd616d1fcb72bc0dd48"), "name" : "测试-C", "course" : [ "语文", "数学", "英语", "科学" ] }
{ "_id" : ObjectId("5bea316f16d1fcb72bc0dd49"), "name" : "测试-D", "course" : [ "数学", "英语", "科学" ] }

  • 查询同时参加语文和数学课程的学生：
    现在两个数组内容都需要保存，所以使用”{‘$all’:[内容1,内容2]}”

db.students.find({'course':{'$all':['语文','数学']}})
{ "_id" : ObjectId("5be93fb416d1fcb72bc0dd46"), "name" : "测试-A", "course" : [ "语文", "数学", "英语" ] }
{ "_id" : ObjectId("5be93fc016d1fcb72bc0dd47"), "name" : "测试-B", "course" : [ "语文", "数学" ] }
{ "_id" : ObjectId("5be93fd616d1fcb72bc0dd48"), "name" : "测试-C", "course" : [ "语文", "数学", "英语", "科学" ] }

  • 虽然‘$all’可以用于数组上，但是也可以用于一个数据的匹配，例如查询在拱墅区的学生：
    这两种查询结果是一样的：

db.students.find({'address':{'$all':['拱墅区']}})
{ "_id" : ObjectId("5be9274916d1fcb72bc0dd42"), "name" : "李思", "sex" : "男", "age" : 20, "score" : 69, "address" : "拱墅区" }
db.students.find({'address':'拱墅区'})
{ "_id" : ObjectId("5be9274916d1fcb72bc0dd42"), "name" : "李思", "sex" : "男", "age" : 20, "score" : 69, "address" : "拱墅区" }

  • 既然在集合中保存的是数组信息，那么数组就可以利用索引操作，使用”key.index”的方式来定义索引：
    例如查询数组中第二个内容是英语的学生：（index = 1）

db.students.find({'course.1':'英语'})
{ "_id" : ObjectId("5bea316f16d1fcb72bc0dd49"), "name" : "测试-D", "course" : [ "数学", "英语", "科学" ] }

使用“$size”来进行数量的控制：
例如查询出只参加四门课程的学生：

db.students.find({'course':{'$size':4}})
{ "_id" : ObjectId("5be93fd616d1fcb72bc0dd48"), "name" : "测试-C", "course" : [ "语文", "数学", "英语", "科学" ] }

现在有个问题：只要条件满足，数组的内容就全部显示出来了，如果不想要全部显示呢？

  • 现在希望可以控制数组返回的数量，那么可以用“$slice”进行控制：
    例如查询出名字叫测试-C的学生，并且把他的课程显示控制为2门：

db.students.find({'name':'测试-C'},{'course':{'$slice':2}})
{ "_id" : ObjectId("5be93fd616d1fcb72bc0dd48"), "name" : "测试-C", "course" : [ "语文", "数学" ] }

如果想要控制取后两门，学过js语法的都知道用负号：

db.students.find({'name':'测试-C'},{'course':{'$slice':-2}})
{ "_id" : ObjectId("5be93fd616d1fcb72bc0dd48"), "name" : "测试-C", "course" : [ "英语", "科学" ] }

那么取中间两门课程：

db.students.find({'name':'测试-C'},{'course':{'$slice':[1,2]}})
{ "_id" : ObjectId("5be93fd616d1fcb72bc0dd48"), "name" : "测试-C", "course" : [ "数学", "英语" ] }

这里的1，2表示的是从1index开始返回，返回2count个元素。

  • 嵌套集合查询：
    在mongo数据库中每一个集合数据可以继续保存其他集合的数据，比如学生信息中需要保存家长信息。
    现在有如下数据：
{    "_id" : ObjectId("5bea488e16d1fcb72bc0dd4a"),    "name" : "王木木-A",    "sex" : "男",    "age" : 19,    "score" : 89,    "address" : "西湖区",    "course" : [        "语文",        "数学"    >],    "parents" : [        {            "name" : "王木木-A-父亲",            "age" : 55        },        {            "name" : "王木木-A-母亲",            "age" : 56        }    ]}{    "_id" : ObjectId("5bea48ed16d1fcb72bc0dd4b"),    "name" : "王木木-B",    "sex" : "男",    "age" : 19,    "score" : 89,    "address" : "西湖区",    "course" : [        "语文",        "英语"    ],    "parents" : [        {            "name" : "王木木-B-父亲",            "age" : 51        },        {            "name" : "王木木-B-母亲",            "age" : 53        }    ]}
  • 这种嵌套的集合数据判断只能通过“$elemMatch”查询。
    例如要查询出父母有人年龄是51岁的同学的信息：

db.students.find({'parents':{'$elemMatch':{'age':51}}})
{ "_id" : ObjectId("5bea48ed16d1fcb72bc0dd4b"), "name" : "王木木-B", "sex" : "男", "age" : 19, "score" : 89, "address" : "西湖区", "course" : [ "语文", "英语" ], "parents" : [ { "name" : "王木木-B-父亲", "age" : 51 }, { "name" : "王木木-B-母亲", "age" : 53 } ] }

  • 判断某个字段是否存在
    使用“$exists”可以判断某个字段是否存在，如果设置为true表示存在，false为不存在。
    例如查询带有parents字段的数据：

db.students.find({'parents':{'$exists':true}})
{ "_id" : ObjectId("5bea488e16d1fcb72bc0dd4a"), "name" : "王木木-A", "sex" : "男", "age" : 19, "score" : 89, "address" : "西湖区", "course" : [ "语文", "数学" ], "parents" : [ { "name" : "王木木-A-父亲", "age" : 55 }, { "name" : "王木木-A-母亲", "age" : 56 } ] }
{ "_id" : ObjectId("5bea48ed16d1fcb72bc0dd4b"), "name" : "王木木-B", "sex" : "男", "age" : 19, "score" : 89, "address" : "西湖区", "course" : [ "语文", "英语" ], "parents" : [ { "name" : "王木木-B-父亲", "age" : 51 }, { "name" : "王木木-B-母亲", "age" : 53 } ] }
db.students.find({'parents':{'$exists':false}})
{ "_id" : ObjectId("5be9271d16d1fcb72bc0dd41"), "name" : "张三", "sex" : "男", "age" : 19, "score" : 89, "address" : "西湖区" }
{ "_id" : ObjectId("5be9274916d1fcb72bc0dd42"), "name" : "李思", "sex" : "男", "age" : 20, "score" : 69, "address" : "拱墅区" }
{ "_id" : ObjectId("5be9277416d1fcb72bc0dd43"), "name" : "王五", "sex" : "女", "age" : 18, "score" : 100, "address" : "余杭区" }
{ "_id" : ObjectId("5be9278a16d1fcb72bc0dd44"), "name" : "赵柳", "sex" : "女", "age" : 17, "score" : 66, "address" : "余杭区" }
{ "_id" : ObjectId("5be927af16d1fcb72bc0dd45"), "name" : "刘琦", "sex" : "女", "age" : 21, "score" : 0, "address" : "上城区" }
{ "_id" : ObjectId("5be93fb416d1fcb72bc0dd46"), "name" : "测试-A", "course" : [ "语文", "数学", "英语" ] }
{ "_id" : ObjectId("5be93fc016d1fcb72bc0dd47"), "name" : "测试-B", "course" : [ "语文", "数学" ] }
{ "_id" : ObjectId("5be93fd616d1fcb72bc0dd48"), "name" : "测试-C", "course" : [ "语文", "数学", "英语", "科学" ] }
{ "_id" : ObjectId("5bea316f16d1fcb72bc0dd49"), "name" : "测试-D", "course" : [ "数学", "英语", "科学" ] }

可以利用这种查询来过滤操作。

*** 条件过滤：**
传统关系型数据库对于数据的筛选，首先想到的是where，MongoDB中也有“$where”：
例如查询出所有年龄大于19岁的数据：

db.students.find({'$where':'this.age>19'})
{ "_id" : ObjectId("5be9274916d1fcb72bc0dd42"), "name" : "李思", "sex" : "男", "age" : 20, "score" : 69, "address" : "拱墅区" }
{ "_id" : ObjectId("5be927af16d1fcb72bc0dd45"), "name" : "刘琦", "sex" : "女", "age" : 21, "score" : 0, "address" : "上城区" }
还可以更简化写法：
db.students.find('this.age>19')
{ "_id" : ObjectId("5be9274916d1fcb72bc0dd42"), "name" : "李思", "sex" : "男", "age" : 20, "score" : 69, "address" : "拱墅区" }
{ "_id" : ObjectId("5be927af16d1fcb72bc0dd45"), "name" : "刘琦", "sex" : "女", "age" : 21, "score" : 0, "address" : "上城区" }

可以看出对于where是可以简化的。但是这类操作是属于对于每一行信息进行判断，对于数据量大的情况并不方便使用。
上面代码严格来讲是属于编写一个操作函数。

  • 上面只查询了一个条件，如果是多个条件的写法是：

db.students.find({'$and':[{'$where':'this.age>19'},{'$where':'this.age<21'}]})
{ "_id" : ObjectId("5be9274916d1fcb72bc0dd42"), "name" : "李思", "sex" : "男", "age" : 20, "score" : 69, "address" : "拱墅区" }

虽然这种形式可以实现数据查询，但是最大缺点是将在MGO数据库里面保存的BSON数据变为了JS的语法结构，这样的方式不方便实用数据库索引机制。

  • 正则运算：
    要想实现模糊查询，必须使用正则表达式，mgDB的正则是与perl兼容的正则表达式形式。
    基础语法：{key:正则标记}
    完整语法：{key:{‘$regex’:正则标记, ‘$options’:选项}}
    对于options主要是设置时正则的信息查询的标记：
    i：忽略字母大小写；
    m：多行查找
    x：空白字符除了被转义的或在字符类中以外的完全被忽略
    s：匹配所有的字符，包括换行内容
    例如查询以“测”字开头的姓名的数据：

db.students.find({'name':/测/})
{ "_id" : ObjectId("5be93fb416d1fcb72bc0dd46"), "name" : "测试-A", "course" : [ "语文", "数学", "英语" ] }
{ "_id" : ObjectId("5be93fc016d1fcb72bc0dd47"), "name" : "测试-B", "course" : [ "语文", "数学" ] }
{ "_id" : ObjectId("5be93fd616d1fcb72bc0dd48"), "name" : "测试-C", "course" : [ "语文", "数学", "英语", "科学" ] }
{ "_id" : ObjectId("5bea316f16d1fcb72bc0dd49"), "name" : "测试-D", "course" : [ "数学", "英语", "科学" ] }

完整写法：

db.students.find({'name':{'$regex':/测/}})
{ "_id" : ObjectId("5be93fb416d1fcb72bc0dd46"), "name" : "测试-A", "course" : [ "语文", "数学", "英语" ] }
{ "_id" : ObjectId("5be93fc016d1fcb72bc0dd47"), "name" : "测试-B", "course" : [ "语文", "数学" ] }
{ "_id" : ObjectId("5be93fd616d1fcb72bc0dd48"), "name" : "测试-C", "course" : [ "语文", "数学", "英语", "科学" ] }
{ "_id" : ObjectId("5bea316f16d1fcb72bc0dd49"), "name" : "测试-D", "course" : [ "数学", "英语", "科学" ] }

  • 上面是用正则查单个数据，现在来用正则查数组数据：

db.students.find({'course':/科/})
{ "_id" : ObjectId("5be93fd616d1fcb72bc0dd48"), "name" : "测试-C", "course" : [ "语文", "数学", "英语", "科学" ] }
{ "_id" : ObjectId("5bea316f16d1fcb72bc0dd49"), "name" : "测试-D", "course" : [ "数学", "英语", "科学" ] }

  • 数据排序
    在MongoDB中数据排序只用sort()函数，可以有两个顺序，升(1)、降(-1)
    例如把学生年龄做个排序：

db.students.find().sort({'age':-1})
{ "_id" : ObjectId("5be927af16d1fcb72bc0dd45"), "name" : "刘琦", "sex" : "女", "age" : 21, "score" : 0, "address" : "上城区" }
{ "_id" : ObjectId("5be9274916d1fcb72bc0dd42"), "name" : "李思", "sex" : "男", "age" : 20, "score" : 69, "address" : "拱墅区" }
{ "_id" : ObjectId("5be9271d16d1fcb72bc0dd41"), "name" : "张三", "sex" : "男", "age" : 19, "score" : 89, "address" : "西湖区" }
{ "_id" : ObjectId("5bea488e16d1fcb72bc0dd4a"), "name" : "王木木-A", "sex" : "男", "age" : 19, "score" : 89, "address" : "西湖区", "course" : [ "语文", "数学" ], "parents" : [ { "name" : "王木木-A-父亲", "age" : 55 }, { "name" : "王木木-A-母亲", "age" : 56 } ] }
{ "_id" : ObjectId("5bea48ed16d1fcb72bc0dd4b"), "name" : "王木木-B", "sex" : "男", "age" : 19, "score" : 89, "address" : "西湖区", "course" : [ "语文", "英语" ], "parents" : [ { "name" : "王木木-B-父亲", "age" : 51 }, { "name" : "王木木-B-母亲", "age" : 53 } ] }
{ "_id" : ObjectId("5be9277416d1fcb72bc0dd43"), "name" : "王五", "sex" : "女", "age" : 18, "score" : 100, "address" : "余杭区" }
{ "_id" : ObjectId("5be9278a16d1fcb72bc0dd44"), "name" : "赵柳", "sex" : "女", "age" : 17, "score" : 66, "address" : "余杭区" }
{ "_id" : ObjectId("5be93fb416d1fcb72bc0dd46"), "name" : "测试-A", "course" : [ "语文", "数学", "英语" ] }
{ "_id" : ObjectId("5be93fc016d1fcb72bc0dd47"), "name" : "测试-B", "course" : [ "语文", "数学" ] }
{ "_id" : ObjectId("5be93fd616d1fcb72bc0dd48"), "name" : "测试-C", "course" : [ "语文", "数学", "英语", "科学" ] }
{ "_id" : ObjectId("5bea316f16d1fcb72bc0dd49"), "name" : "测试-D", "course" : [ "数学", "英语", "科学" ] }

  • 还有一种排序方式叫做自然排序，就是按照数据保存的先后顺序来排序：$natural

db.students.find().sort({'$natural':-1})
{ "_id" : ObjectId("5bea48ed16d1fcb72bc0dd4b"), "name" : "王木木-B", "sex" : "男", "age" : 19, "score" : 89, "address" : "西湖区", "course" : [ "语文", "英语" ], "parents" : [ { "name" : "王木木-B-父亲", "age" : 51 }, { "name" : "王木木-B-母亲", "age" : 53 } ] }
{ "_id" : ObjectId("5bea488e16d1fcb72bc0dd4a"), "name" : "王木木-A", "sex" : "男", "age" : 19, "score" : 89, "address" : "西湖区", "course" : [ "语文", "数学" ], "parents" : [ { "name" : "王木木-A-父亲", "age" : 55 }, { "name" : "王木木-A-母亲", "age" : 56 } ] }
{ "_id" : ObjectId("5bea316f16d1fcb72bc0dd49"), "name" : "测试-D", "course" : [ "数学", "英语", "科学" ] }
{ "_id" : ObjectId("5be93fd616d1fcb72bc0dd48"), "name" : "测试-C", "course" : [ "语文", "数学", "英语", "科学" ] }
{ "_id" : ObjectId("5be93fc016d1fcb72bc0dd47"), "name" : "测试-B", "course" : [ "语文", "数学" ] }
{ "_id" : ObjectId("5be93fb416d1fcb72bc0dd46"), "name" : "测试-A", "course" : [ "语文", "数学", "英语" ] }
{ "_id" : ObjectId("5be927af16d1fcb72bc0dd45"), "name" : "刘琦", "sex" : "女", "age" : 21, "score" : 0, "address" : "上城区" }
{ "_id" : ObjectId("5be9278a16d1fcb72bc0dd44"), "name" : "赵柳", "sex" : "女", "age" : 17, "score" : 66, "address" : "余杭区" }
{ "_id" : ObjectId("5be9277416d1fcb72bc0dd43"), "name" : "王五", "sex" : "女", "age" : 18, "score" : 100, "address" : "余杭区" }
{ "_id" : ObjectId("5be9274916d1fcb72bc0dd42"), "name" : "李思", "sex" : "男", "age" : 20, "score" : 69, "address" : "拱墅区" }
{ "_id" : ObjectId("5be9271d16d1fcb72bc0dd41"), "name" : "张三", "sex" : "男", "age" : 19, "score" : 89, "address" : "西湖区" }

  • 数据分页

skip(n)：表示跨过多少数据
limit(n)：表示取出的数据行的个数限制
例如：分页显示第一页，每页显示2条：skip(0)，limit(2)：
》db.students.find().sort({'$natural':1}).skip(0).limit(2)
{ "_id" : ObjectId("5be9271d16d1fcb72bc0dd41"), "name" : "张三", "sex" : "男", "age" : 19, "score" : 89, "address" : "西湖区" }
{ "_id" : ObjectId("5be9274916d1fcb72bc0dd42"), "name" : "李思", "sex" : "男", "age" : 20, "score" : 69, "address" : "拱墅区" }

第二页，每页显示2条：

db.students.find().sort({'$natural':1}).skip(2).limit(2)
{ "_id" : ObjectId("5be9277416d1fcb72bc0dd43"), "name" : "王五", "sex" : "女", "age" : 18, "score" : 100, "address" : "余杭区" }
{ "_id" : ObjectId("5be9278a16d1fcb72bc0dd44"), "name" : "赵柳", "sex" : "女", "age" : 17, "score" : 66, "address" : "余杭区" }

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