Mongodb基础实践（二）
admin
2023-05-23 21:42:55
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在前面的文章里面主要介绍了MongoDB的文档，集合，数据库等操作和对文档的增、删、改相关知识，接下来会总结一点有关查询的相关知识。
在MySQL中,我们知道数据查询是优化的主要内容，读写分离等技术都是可以用来处理数据库查询优化的，足以见数据库查询是每个系统中很重要的一部分，之前介绍了find的简单使用，下面会介绍一些相对比较复杂一点的查询。
一、数据查询
MySQL数据库中主要是用select 结合where子句实现数据的查询，功能特别强大，例如多表联合查询、支持正则表达式等。不在这里做过多的相关介绍。这里主要介绍MongoDB的相关查询，MongoDB中主要用find()实现数据的查询，同时也可以使用一些条件限制。
1.1显示单条数据
在上篇文章中提到了find()的使用，但是每次查询数据，都是查询所有的，显示其中的一部分，可以用it迭代。有时候我们想要查询其中的一条数据，具体操作要根据具体需求实现。
MongoDB 查询数据的语法
db.collection.find(query, projection)
query ：可选，使用查询操作符指定查询条件
projection ：可选，使用投影操作符指定返回的键。查询时返回文档中所有键值， 只需省略该参数即可（默认省略）。可以使用 pretty() 方法以易读的方式来读取数据，，语法格式如下
>db.col.find().pretty()
pretty() 方法以格式化的方式来显示所有文档。
例如：
db.winner.find().pretty()
{ "_id" : ObjectId("592e7d1caaa464fa8a557e95"), "winne" : 1955 }
{ "_id" : ObjectId("592e7d1eaaa464fa8a557e96"), "winne" : 1955 }
{ "_id" : ObjectId("592e7d1faaa464fa8a557e97"), "winne" : 1955 }
{ "_id" : ObjectId("592e7d1faaa464fa8a557e98"), "winne" : 1955 }
{ "_id" : ObjectId("592e7d21aaa464fa8a557e99"), "winne" : 1955 }
{ "_id" : ObjectId("592e7d22aaa464fa8a557e9a"), "winne" : 1955 }
{ "_id" : ObjectId("592e7e14aaa464fa8a557ec4"), "winne" : 41 }
{ "_id" : ObjectId("592e7e14aaa464fa8a557ec5"), "winne" : 42 }
{ "_id" : ObjectId("592e7e14aaa464fa8a557ec6"), "winne" : 43 }
{ "_id" : ObjectId("592e7e14aaa464fa8a557ec7"), "winne" : 44 }1、查询某个集合中的所有数据
db.winner.find()
{ "_id" : ObjectId("592e7d1caaa464fa8a557e95"), "winne" : 1955 }
{ "_id" : ObjectId("592e7d1eaaa464fa8a557e96"), "winne" : 1955 }
{ "_id" : ObjectId("592e7d1faaa464fa8a557e97"), "winne" : 1955 }
{ "_id" : ObjectId("592e7d1faaa464fa8a557e98"), "winne" : 1955 }
{ "_id" : ObjectId("592e7d21aaa464fa8a557e99"), "winne" : 1955 }
{ "_id" : ObjectId("592e7d22aaa464fa8a557e9a"), "winne" : 1955 }
{ "_id" : ObjectId("592e7e14aaa464fa8a557ec4"), "winne" : 41 }
{ "_id" : ObjectId("592e7e14aaa464fa8a557ec5"), "winne" : 42 }
{ "_id" : ObjectId("592e7e14aaa464fa8a557ec6"), "winne" : 43 }
{ "_id" : ObjectId("592e7e14aaa464fa8a557ec7"), "winne" : 44 }
{ "_id" : ObjectId("592e7e14aaa464fa8a557ec8"), "winne" : 45 }
{ "_id" : ObjectId("592e7e14aaa464fa8a557ec9"), "winne" : 46 }
{ "_id" : ObjectId("592e7e14aaa464fa8a557eca"), "winne" : 47 }
{ "_id" : ObjectId("592e7e14aaa464fa8a557ecb"), "winne" : 48 }
{ "_id" : ObjectId("592e7e14aaa464fa8a557ecc"), "winne" : 49 }
{ "_id" : ObjectId("592e7e14aaa464fa8a557ecd"), "winne" : 50 }
{ "_id" : ObjectId("592e7e14aaa464fa8a557ece"), "winne" : 51 }
{ "_id" : ObjectId("592e7e14aaa464fa8a557ecf"), "winne" : 52 }
{ "_id" : ObjectId("592e7e14aaa464fa8a557ed0"), "winne" : 53 }
{ "_id" : ObjectId("592e7e14aaa464fa8a557ed1"), "winne" : 54 }
Type "it" for more默认显示20条数据，其他数据可以输入it迭代。
2、显示一条数据
find()是输出所有结果，里面可能有些文档内容相同，但是“_id”肯定是不一样的，这时我们可以使用findOne()方法查询，或者可以使用db.winner.find({winne:1955}).limit(1)。
db.winner.find({winne:1955})
{ "_id" : ObjectId("592e7d1caaa464fa8a557e95"), "winne" : 1955 }
{ "_id" : ObjectId("592e7d1eaaa464fa8a557e96"), "winne" : 1955 }
{ "_id" : ObjectId("592e7d1faaa464fa8a557e97"), "winne" : 1955 }
{ "_id" : ObjectId("592e7d1faaa464fa8a557e98"), "winne" : 1955 }
{ "_id" : ObjectId("592e7d21aaa464fa8a557e99"), "winne" : 1955 }
{ "_id" : ObjectId("592e7d22aaa464fa8a557e9a"), "winne" : 1955 }
假如要查询winner集合中winne=1955的一条数据，而用find()查询出所有的数据，这时就可以使用findOne()
db.winner.findOne({winne:1955})
{ "_id" : ObjectId("592e7d1caaa464fa8a557e95"), "winne" : 1955 }
或者可以使用
db.winner.find({winne:1955}).limit(1)
{ "_id" : ObjectId("592e7d1caaa464fa8a557e95"), "winne" : 1955 }
两者的区别
findOne()有点类似MySQL里面的distinct,会返回查询的第一条结果，如果搜索不到想要的数据就会
返回NULL，
db.winner.findOne({winne:200888})
null
db.winner.find({winne:1955}).limit(1)方法就和MySQL里面的limit是一样的，主要是限制查询结果的条数。3、查询满足一定条件的数据
在MySQL中查询时，可以结合where以及字段等信息查询数据，而MongoDB中也是可以的，同样可以支持一些条件判断语句。
| 格式 | 范例 | RDBMS中的类似语句 | |
|---|---|---|---|
| 等于 | {} | db.col.find({"winne":"1995"}).pretty() | where |
| 小于 | { | db.col.find({" | where |
| 小于或等于 | { | db.col.find({" | where |
| 大于 | { | db.col.find({" | where |
| 大于或等于 | { | db.col.find({" | where |
| 不等于 | { | db.col.find({" | where |
$gt -------- greater than $gte --------- gt equal
$lt -------- less than $lte --------- lt equal
$ne ----------- not equal
1、查询winner集合中winne<50的相关数据
db.winner.find({winne:{$lt :50}})
{ "_id" : ObjectId("592e7e14aaa464fa8a557ec4"), "winne" : 41 }
{ "_id" : ObjectId("592e7e14aaa464fa8a557ec5"), "winne" : 42 }
{ "_id" : ObjectId("592e7e14aaa464fa8a557ec6"), "winne" : 43 }
{ "_id" : ObjectId("592e7e14aaa464fa8a557ec7"), "winne" : 44 }
{ "_id" : ObjectId("592e7e14aaa464fa8a557ec8"), "winne" : 45 }
{ "_id" : ObjectId("592e7e14aaa464fa8a557ec9"), "winne" : 46 }
{ "_id" : ObjectId("592e7e14aaa464fa8a557eca"), "winne" : 47 }
{ "_id" : ObjectId("592e7e14aaa464fa8a557ecb"), "winne" : 48 }
{ "_id" : ObjectId("592e7e14aaa464fa8a557ecc"), "winne" : 49 }
{ "_id" : ObjectId("592e7e17aaa464fa8a557f28"), "winne" : 41 }
{ "_id" : ObjectId("592e7e17aaa464fa8a557f29"), "winne" : 42 }
{ "_id" : ObjectId("592e7e17aaa464fa8a557f2a"), "winne" : 43 }
{ "_id" : ObjectId("592e7e17aaa464fa8a557f2b"), "winne" : 44 }
2、查询winner集合中40=4、MongoDB AND 条件
MongoDB 的 find() 方法可以传入多个键(key)，每个键(key)以逗号隔开，语法格式如下：
>db.winner.find({key1:value1, key2:value2}).pretty()
#插入测试数据
for(i=0;i<20;i++)db.info2.insert({name:"linux",
object:"SA",
company:"docker",
phone:i})
for(i=0;i<20;i++)db.info2.insert({name:"openstack",
object:"DBA",
company:"could",
phone:i})
#检查测试数据
> db.info2.find()
db.info2.find()
{ "_id" : ObjectId("592f838dd276944818f7edb4"), "name" : "linux", "object" : "SA", "company" : "docker", "phone" : 0 }
{ "_id" : ObjectId("592f838dd276944818f7edb5"), "name" : "linux", "object" : "SA", "company" : "docker", "phone" : 1 }
{ "_id" : ObjectId("592f838dd276944818f7edb6"), "name" : "linux", "object" : "SA", "company" : "docker", "phone" : 2 }
{ "_id" : ObjectId("592f838dd276944818f7edb7"), "name" : "linux", "object" : "SA", "company" : "docker", "phone" : 3 }
{ "_id" : ObjectId("592f838dd276944818f7edb8"), "name" : "linux", "object" : "SA", "company" : "docker", "phone" : 4 }
{ "_id" : ObjectId("592f838dd276944818f7edb9"), "name" : "linux", "object" : "SA", "company" : "docker", "phone" : 5 }
{ "_id" : ObjectId("592f838dd276944818f7edba"), "name" : "linux", "object" : "SA", "company" : "docker", "phone" : 6 }
{ "_id" : ObjectId("592f838dd276944818f7edbb"), "name" : "linux", "object" : "SA", "company" : "docker", "phone" : 7 }
{ "_id" : ObjectId("592f838dd276944818f7edbc"), "name" : "linux", "object" : "SA", "company" : "docker", "phone" : 8 }
{ "_id" : ObjectId("592f838dd276944818f7edbd"), "name" : "linux", "object" : "SA", "company" : "docker", "phone" : 9 }
{ "_id" : ObjectId("592f838dd276944818f7edbe"), "name" : "linux", "object" : "SA", "company" : "docker", "phone" : 10 }
{ "_id" : ObjectId("592f838dd276944818f7edbf"), "name" : "linux", "object" : "SA", "company" : "docker", "phone" : 11 }
{ "_id" : ObjectId("592f838dd276944818f7edc0"), "name" : "linux", "object" : "SA", "company" : "docker", "phone" : 12 }
{ "_id" : ObjectId("592f838dd276944818f7edc1"), "name" : "linux", "object" : "SA", "company" : "docker", "phone" : 13 }
{ "_id" : ObjectId("592f838dd276944818f7edc2"), "name" : "linux", "object" : "SA", "company" : "docker", "phone" : 14 }
{ "_id" : ObjectId("592f838dd276944818f7edc3"), "name" : "linux", "object" : "SA", "company" : "docker", "phone" : 15 }
{ "_id" : ObjectId("592f838dd276944818f7edc4"), "name" : "linux", "object" : "SA", "company" : "docker", "phone" : 16 }
{ "_id" : ObjectId("592f838dd276944818f7edc5"), "name" : "linux", "object" : "SA", "company" : "docker", "phone" : 17 }
{ "_id" : ObjectId("592f838dd276944818f7edc6"), "name" : "linux", "object" : "SA", "company" : "docker", "phone" : 18 }
{ "_id" : ObjectId("592f838dd276944818f7edc7"), "name" : "linux", "object" : "SA", "company" : "docker", "phone" : 19 }
>db.info2.find().count() #检查数据的条数
40
#筛选name=linux object=SA phone<5
db.info2.find({name:"linux",object:"SA",phone:{$lt:5}})
执行
db.info2.find({name:"linux",object:"SA",phone:{$lt:5}})
{ "_id" : ObjectId("592f838dd276944818f7edb4"), "name" : "linux", "object" : "SA", "company" : "docker", "phone" : 0 }
{ "_id" : ObjectId("592f838dd276944818f7edb5"), "name" : "linux", "object" : "SA", "company" : "docker", "phone" : 1 }
{ "_id" : ObjectId("592f838dd276944818f7edb6"), "name" : "linux", "object" : "SA", "company" : "docker", "phone" : 2 }
{ "_id" : ObjectId("592f838dd276944818f7edb7"), "name" : "linux", "object" : "SA", "company" : "docker", "phone" : 3 }
{ "_id" : ObjectId("592f838dd276944818f7edb8"), "name" : "linux", "object" : "SA", "company" : "docker", "phone" : 4 }
筛选name=linux object=SA 55 MongoDB OR 条件
MongoDB 除了有类似MySQL的AND条件语句外，还有OR 条件语句，OR 条件语句使用了关键字 $or,语法格式如下：
>db.collections.find(
{
$or: [
{key1: value1}, {key2:value2}
]
}
).pretty()
#查name=linux 或者object=redis
db.info2.find(
{$or:[{name:"linux"},{object:"redis"}]
}
)
db.info2.find(db.info2.find(
... {$or:[{name:"linux"},{object:"redis"}]{$or:[{name:"linux"},{object:"redis"}]
...
... }}
... ))
{ "_id" : ObjectId("592f838dd276944818f7edb4"), "name" : "linux", "object" : "SA", "company" : "docker", "phone" : 0 }
{ "_id" : ObjectId("592f838dd276944818f7edb5"), "name" : "linux", "object" : "SA", "company" : "docker", "phone" : 1 }
{ "_id" : ObjectId("592f838dd276944818f7edb6"), "name" : "linux", "object" : "SA", "company" : "docker", "phone" : 2 }
{ "_id" : ObjectId("592f838dd276944818f7edb7"), "name" : "linux", "object" : "SA", "company" : "docker", "phone" : 3 }
{ "_id" : ObjectId("592f838dd276944818f7edb8"), "name" : "linux", "object" : "SA", "company" : "docker", "phone" : 4 }
{ "_id" : ObjectId("592f838dd276944818f7edb9"), "name" : "linux", "object" : "SA", "company" : "docker", "phone" : 5 }
{ "_id" : ObjectId("592f838dd276944818f7edba"), "name" : "linux", "object" : "SA", "company" : "docker", "phone" : 6 }
{ "_id" : ObjectId("592f838dd276944818f7edbb"), "name" : "linux", "object" : "SA", "company" : "docker", "phone" : 7 }
{ "_id" : ObjectId("592f838dd276944818f7edbc"), "name" : "linux", "object" : "SA", "company" : "docker", "phone" : 8 }
{ "_id" : ObjectId("592f838dd276944818f7edbd"), "name" : "linux", "object" : "SA", "company" : "docker", "phone" : 9 }
{ "_id" : ObjectId("592f838dd276944818f7edbe"), "name" : "linux", "object" : "SA", "company" : "docker", "phone" : 10 }
{ "_id" : ObjectId("592f838dd276944818f7edbf"), "name" : "linux", "object" : "SA", "company" : "docker", "phone" : 11 }
{ "_id" : ObjectId("592f838dd276944818f7edc0"), "name" : "linux", "object" : "SA", "company" : "docker", "phone" : 12 }
{ "_id" : ObjectId("592f838dd276944818f7edc1"), "name" : "linux", "object" : "SA", "company" : "docker", "phone" : 13 }
{ "_id" : ObjectId("592f838dd276944818f7edc2"), "name" : "linux", "object" : "SA", "company" : "docker", "phone" : 14 }
{ "_id" : ObjectId("592f838dd276944818f7edc3"), "name" : "linux", "object" : "SA", "company" : "docker", "phone" : 15 }
{ "_id" : ObjectId("592f838dd276944818f7edc4"), "name" : "linux", "object" : "SA", "company" : "docker", "phone" : 16 }
{ "_id" : ObjectId("592f838dd276944818f7edc5"), "name" : "linux", "object" : "SA", "company" : "docker", "phone" : 17 }
{ "_id" : ObjectId("592f838dd276944818f7edc6"), "name" : "linux", "object" : "SA", "company" : "docker", "phone" : 18 }
{ "_id" : ObjectId("592f838dd276944818f7edc7"), "name" : "linux", "object" : "SA", "company" : "docker", "phone" : 19 }
Type "it" for more
> itit
{ "_id" : ObjectId("592f8964d276944818f7eddc"), "name" : "MongoDB", "object" : "redis", "company" : "winner", "phone" : 0 }
{ "_id" : ObjectId("592f8964d276944818f7eddd"), "name" : "MongoDB", "object" : "redis", "company" : "winner", "phone" : 1 }
{ "_id" : ObjectId("592f8964d276944818f7edde"), "name" : "MongoDB", "object" : "redis", "company" : "winner", "phone" : 2 }
{ "_id" : ObjectId("592f8964d276944818f7eddf"), "name" : "MongoDB", "object" : "redis", "company" : "winner", "phone" : 3 }
{ "_id" : ObjectId("592f8964d276944818f7ede0"), "name" : "MongoDB", "object" : "redis", "company" : "winner", "phone" : 4 }
{ "_id" : ObjectId("592f8964d276944818f7ede1"), "name" : "MongoDB", "object" : "redis", "company" : "winner", "phone" : 5 }
{ "_id" : ObjectId("592f8964d276944818f7ede2"), "name" : "MongoDB", "object" : "redis", "company" : "winner", "phone" : 6 }
{ "_id" : ObjectId("592f8964d276944818f7ede3"), "name" : "MongoDB", "object" : "redis", "company" : "winner", "phone" : 7 }
{ "_id" : ObjectId("592f8964d276944818f7ede4"), "name" : "MongoDB", "object" : "redis", "company" : "winner", "phone" : 8 }
{ "_id" : ObjectId("592f8964d276944818f7ede5"), "name" : "MongoDB", "object" : "redis", "company" : "winner", "phone" : 9 }6、AND和OR综合使用
查询phone<5,name=MongoDB或者name=linux
db.info2.find({phone:{$lt:5},$or:[{name:"MongoDB"},{name:"linux"}]})
db.info2.find({phone:{$lt:5},$or:[{name:"MongoDB"},{name:"linux"}]})
{ "_id" : ObjectId("592f838dd276944818f7edb4"), "name" : "linux", "object" : "SA", "company" : "docker", "phone" : 0 }
{ "_id" : ObjectId("592f838dd276944818f7edb5"), "name" : "linux", "object" : "SA", "company" : "docker", "phone" : 1 }
{ "_id" : ObjectId("592f838dd276944818f7edb6"), "name" : "linux", "object" : "SA", "company" : "docker", "phone" : 2 }
{ "_id" : ObjectId("592f838dd276944818f7edb7"), "name" : "linux", "object" : "SA", "company" : "docker", "phone" : 3 }
{ "_id" : ObjectId("592f838dd276944818f7edb8"), "name" : "linux", "object" : "SA", "company" : "docker", "phone" : 4 }
{ "_id" : ObjectId("592f8964d276944818f7eddc"), "name" : "MongoDB", "object" : "redis", "company" : "winner", "phone" : 0 }
{ "_id" : ObjectId("592f8964d276944818f7eddd"), "name" : "MongoDB", "object" : "redis", "company" : "winner", "phone" : 1 }
{ "_id" : ObjectId("592f8964d276944818f7edde"), "name" : "MongoDB", "object" : "redis", "company" : "winner", "phone" : 2 }
{ "_id" : ObjectId("592f8964d276944818f7eddf"), "name" : "MongoDB", "object" : "redis", "company" : "winner", "phone" : 3 }
{ "_id" : ObjectId("592f8964d276944818f7ede0"), "name" : "MongoDB", "object" : "redis", "company" : "winner", "phone" : 4 }7、查询结果排序
在MySQL中是有order by条件，可以根据desc或者asc进行升序或者降序操作，而MongoDB中是可以利用sort()方法实现排序的，例如对6中的结果处理，根据phone排序。
基本语法
db.info2.find().sort({phone:1}) #这里phone表示根据该key排序，1表示升序，-1表示降序。
db.info2.find({phone:{$lt:5},$or:[{name:"MongoDB"},{name:"linux"}]}).sort({phone:1})
{ "_id" : ObjectId("592f838dd276944818f7edb4"), "name" : "linux", "object" : "SA", "company" : "docker", "phone" : 0 }
{ "_id" : ObjectId("592f8964d276944818f7eddc"), "name" : "MongoDB", "object" : "redis", "company" : "winner", "phone" : 0 }
{ "_id" : ObjectId("592f838dd276944818f7edb5"), "name" : "linux", "object" : "SA", "company" : "docker", "phone" : 1 }
{ "_id" : ObjectId("592f8964d276944818f7eddd"), "name" : "MongoDB", "object" : "redis", "company" : "winner", "phone" : 1 }
{ "_id" : ObjectId("592f838dd276944818f7edb6"), "name" : "linux", "object" : "SA", "company" : "docker", "phone" : 2 }
{ "_id" : ObjectId("592f8964d276944818f7edde"), "name" : "MongoDB", "object" : "redis", "company" : "winner", "phone" : 2 }
{ "_id" : ObjectId("592f838dd276944818f7edb7"), "name" : "linux", "object" : "SA", "company" : "docker", "phone" : 3 }
{ "_id" : ObjectId("592f8964d276944818f7eddf"), "name" : "MongoDB", "object" : "redis", "company" : "winner", "phone" : 3 }
{ "_id" : ObjectId("592f838dd276944818f7edb8"), "name" : "linux", "object" : "SA", "company" : "docker", "phone" : 4 }
{ "_id" : ObjectId("592f8964d276944818f7ede0"), "name" : "MongoDB", "object" : "redis", "company" : "winner", "phone" : 4 }8、MongoDB Skip() 方法
在前面介绍了limit()，sort()，count()等方法，接下来要介绍一个比较有趣的skip()方法，在使用limit()的时候可以显示你要求的几条，而skip()方法是跳过几条。
db.info2.find({phone:{$lt:5},$or:[{name:"MongoDB"},{name:"linux"}]}).sort({phone:1})
{ "_id" : ObjectId("592f838dd276944818f7edb4"), "name" : "linux", "object" : "SA", "company" : "docker", "phone" : 0 }
{ "_id" : ObjectId("592f8964d276944818f7eddc"), "name" : "MongoDB", "object" : "redis", "company" : "winner", "phone" : 0 }
{ "_id" : ObjectId("592f838dd276944818f7edb5"), "name" : "linux", "object" : "SA", "company" : "docker", "phone" : 1 }
{ "_id" : ObjectId("592f8964d276944818f7eddd"), "name" : "MongoDB", "object" : "redis", "company" : "winner", "phone" : 1 }
{ "_id" : ObjectId("592f838dd276944818f7edb6"), "name" : "linux", "object" : "SA", "company" : "docker", "phone" : 2 }
{ "_id" : ObjectId("592f8964d276944818f7edde"), "name" : "MongoDB", "object" : "redis", "company" : "winner", "phone" : 2 }
{ "_id" : ObjectId("592f838dd276944818f7edb7"), "name" : "linux", "object" : "SA", "company" : "docker", "phone" : 3 }
{ "_id" : ObjectId("592f8964d276944818f7eddf"), "name" : "MongoDB", "object" : "redis", "company" : "winner", "phone" : 3 }
{ "_id" : ObjectId("592f838dd276944818f7edb8"), "name" : "linux", "object" : "SA", "company" : "docker", "phone" : 4 }
{ "_id" : ObjectId("592f8964d276944818f7ede0"), "name" : "MongoDB", "object" : "redis", "company" : "winner", "phone" : 4 }
使用skip()方法
db.info2.find({phone:{$lt:5},$or:[{name:"MongoDB"},{name:"linux"}]}).sort({phone:1}).skip(3)
{ "_id" : ObjectId("592f8964d276944818f7eddd"), "name" : "MongoDB", "object" : "redis", "company" : "winner", "phone" : 1 }
{ "_id" : ObjectId("592f838dd276944818f7edb6"), "name" : "linux", "object" : "SA", "company" : "docker", "phone" : 2 }
{ "_id" : ObjectId("592f8964d276944818f7edde"), "name" : "MongoDB", "object" : "redis", "company" : "winner", "phone" : 2 }
{ "_id" : ObjectId("592f838dd276944818f7edb7"), "name" : "linux", "object" : "SA", "company" : "docker", "phone" : 3 }
{ "_id" : ObjectId("592f8964d276944818f7eddf"), "name" : "MongoDB", "object" : "redis", "company" : "winner", "phone" : 3 }
{ "_id" : ObjectId("592f838dd276944818f7edb8"), "name" : "linux", "object" : "SA", "company" : "docker", "phone" : 4 }
{ "_id" : ObjectId("592f8964d276944818f7ede0"), "name" : "MongoDB", "object" : "redis", "company" : "winner", "phone" : 4 }
skip 方法有点类似于MySQL里面的limit之间间隔情况。这里介绍了有关查询的问题，在数据库中，查询是非常重要的一部分，所以介绍的篇幅也是比较多的，后期遇到其他问题也会继续总结输出。
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