MongonDB学习笔记（一）查询

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在前面增删改查的例子里看到几个不知道的函数，这一节来看看他们都是什么意思。mongondb听起来很高大上，单终归是一个数据库，增删改查是核心功能，这一节重点看看他的查询包括符合查询。其实到这里，我感受最深的是mongondb作为关系性数据库的替代品还是很好上手的，至少比redis更容易让我理解。
首先往数据库集合里面插入几条数据。
测试数据：
> db.users.insert({username:"mongo", url:"webinglin.github.io", tags:["mongodb", database","nosql"],likes:999, author:"linwenbin"})> db.users.insert({username:"redis", url:"webinglin.github.io", tags:["redis","database","nosql"],likes:888, author:"linwenbin"})> db.users.insert({username:"spring", url:"webinglin.github.io", tags:["spring","framework"],likes:777, author:"linwenbin"})> db.users.find().pretty(){        "_id" : ObjectId("5574bdabc705777157a515aa"),        "username" : "mongo",        "url" : "webinglin.github.io",        "tags" : [                "mongodb",                "database",                "nosql"        ],        "likes" : 999,        "author" : "linwenbin"}{        "_id" : ObjectId("5574bdd2c705777157a515ab"),        "username" : "redis",        "url" : "webinglin.github.io",        "tags" : [                "redis",                "database",                "nosql"        ],        "likes" : 888,        "author" : "linwenbin"}{        "_id" : ObjectId("5574bdf3c705777157a515ac"),        "username" : "spring",        "url" : "webinglin.github.io",        "tags" : [                "spring",                "framework"        ],        "likes" : 777,        "author" : "linwenbin"}pretty() 方法是对查询结果进行格式化
查询的时候可以带上查询条件,那具体的查询条件怎么使用？
等于
等于操作直接使用 {key:value} 这样的文档形式即可
> db.users.find({username:"mongo"}){ "_id" : ObjectId("5574bdabc705777157a515aa"), "username" : "mongo", "url" : "webinglin.github.io", "tags" : [ "mongodb", "database", "nosql" ], "likes" : 999, "author" : "linwenbin" }>大于
语法： {key : {$gt:value} }
> db.users.find({likes:{$gt:888}}){ "_id" : ObjectId("5574bdabc705777157a515aa"), "username" : "mongo", "url" : "webinglin.github.io", "tags" : [ "mongodb", "database", "nosql" ], "likes" : 999, "author" : "linwenbin" }>大于等于
语法： {key : {$gte:value} }
> db.users.find({likes:{$gte:888}}){ "_id" : ObjectId("5574bdabc705777157a515aa"), "username" : "mongo", "url" : "webinglin.github.io", "tags" : [ "mongodb", "database", "nosql" ], "likes" : 999, "author" : "linwenbin" }{ "_id" : ObjectId("5574bdd2c705777157a515ab"), "username" : "redis", "url" : "webinglin.github.io", "tags" : [ "redis", "database", "nosql" ], "likes" : 888, "author" : "linwenbin" }小于
语法：　{key : {$lt:value} }
> db.users.find({likes:{$lt:888}}){ "_id" : ObjectId("5574bdf3c705777157a515ac"), "username" : "spring", "url" : "webinglin.github.io", "tags" : [ "spring", "framework" ], "likes" : 777, "author" : "linwenbin" }小于等于
语法：　{key : {$lte:value}}
> db.users.find({likes:{$lte:888}}){ "_id" : ObjectId("5574bdd2c705777157a515ab"), "username" : "redis", "url" : "webinglin.github.io", "tags" : [ "redis", "database", "nosql" ], "likes" : 888, "author" : "linwenbin" }{ "_id" : ObjectId("5574bdf3c705777157a515ac"), "username" : "spring", "url" : "webinglin.github.io", "tags" : [ "spring", "framework" ], "likes" : 777, "author" : "linwenbin" }不等于
语法： {key : {$ne:value} }
> db.users.find({likes:{$ne:888}}){ "_id" : ObjectId("5574bdabc705777157a515aa"), "username" : "mongo", "url" : "webinglin.github.io", "tags" : [ "mongodb", "database", "nosql" ], "likes" : 999, "author" : "linwenbin" }{ "_id" : ObjectId("5574bdf3c705777157a515ac"), "username" : "spring", "url" : "webinglin.github.io", "tags" : [ "spring", "framework" ], "likes" : 777, "author" : "linwenbin" }且操作 AND
语法： {key1:value1, key2:value2, key3:value3 …}
> db.users.find({likes:{$gt:777},username:"mongo"}){ "_id" : ObjectId("5574bdabc705777157a515aa"), "username" : "mongo", "url" : "webinglin.github.io", "tags" : [ "mongodb", "database", "nosql" ], "likes" : 999, "author" : "linwenbin" }> db.users.find({likes:{$gt:777}}){ "_id" : ObjectId("5574bdabc705777157a515aa"), "username" : "mongo", "url" : "webinglin.github.io", "tags" : [ "mongodb", "database", "nosql" ], "likes" : 999, "author" : "linwenbin" }{ "_id" : ObjectId("5574bdd2c705777157a515ab"), "username" : "redis", "url" : "webinglin.github.io", "tags" : [ "redis", "database", "nosql" ], "likes" : 888, "author" : "linwenbin" }或操作 OR
语法： { $or: [ {key1: value1}, {key2:value2} ] } 将or条件的所有 {key:value} 都放在 $or 的value中（数组）
> db.users.find({$or:[{username:"mongo"},{username:"redis"}]}){ "_id" : ObjectId("5574bdabc705777157a515aa"), "username" : "mongo", "url" : "webinglin.github.io", "tags" : [ "mongodb", "database", "nosql" ], "likes" : 999, "author" : "linwenbin" }{ "_id" : ObjectId("5574bdd2c705777157a515ab"), "username" : "redis", "url" : "webinglin.github.io", "tags" : [ "redis", "database", "nosql" ], "likes" : 888, "author" : "linwenbin" }复杂条件查询
如何将所有的条件都连起来用呢？
比如我们想要这样查询 like>=888 && (username=”mongo” or username=”spring”)
由于上面的数据只有三条， 我们知道 like>=888 只有 mongo 和 redis 这两条数据满足条件， 后面的username=”mongo” or username=”spring” 又有 mongo和 spring 满足条件， 这两个and操作之后 就只剩下 mongo 这条数据满足条件了。 所以最终应该查出一条mongo的Document.
> db.users.find({likes:{$gte:888},$or:[{username:"mongo"},{username:"spring"}]}){ "_id" : ObjectId("5574bdabc705777157a515aa"), "username" : "mongo", "url" : "webinglin.github.io", "tags" : [ "mongodb", "database", "nosql" ], "likes" : 999, "author" : "linwenbin" }>find() 其他用法
Projection
mongodb中 projection 意味着显示你希望看到的字段而非所有的字段都显示,这是什么意思呢？
比如： 我们的测试数据里面有那么多的字段: username,likes,tags,author,url 而我们经常要用到的就只有 username 和 likes 那么就显示这两个字段就好了，其他的字段就别显示出来了。
find({},{KEY:1/0}) find的第二个参数，KEY为要显示或隐藏的字段，value为1表示显示，0表示隐藏，看着也很简单，试一下吧
> db.users.find({},{_id:0,url:0,tags:0,author:0}){ "username" : "mongo", "likes" : 999 }{ "username" : "redis", "likes" : 888 }{ "username" : "spring", "likes" : 777 }>limit, skip, sort
为了更好的测试分页的效果，新建一个集合，并插入30条数据
> for(var i=0; i<30; i++){... db.pages.insert({"val":i});... }WriteResult({ "nInserted" : 1 })> db.pages.find(){ "_id" : ObjectId("5574ca7b192e9dda0925e37f"), "val" : 0 }{ "_id" : ObjectId("5574ca7b192e9dda0925e380"), "val" : 1 }{ "_id" : ObjectId("5574ca7b192e9dda0925e381"), "val" : 2 }{ "_id" : ObjectId("5574ca7b192e9dda0925e382"), "val" : 3 }{ "_id" : ObjectId("5574ca7b192e9dda0925e383"), "val" : 4 }{ "_id" : ObjectId("5574ca7b192e9dda0925e384"), "val" : 5 }{ "_id" : ObjectId("5574ca7b192e9dda0925e385"), "val" : 6 }{ "_id" : ObjectId("5574ca7b192e9dda0925e386"), "val" : 7 }{ "_id" : ObjectId("5574ca7b192e9dda0925e387"), "val" : 8 }{ "_id" : ObjectId("5574ca7b192e9dda0925e388"), "val" : 9 }{ "_id" : ObjectId("5574ca7b192e9dda0925e389"), "val" : 10 }{ "_id" : ObjectId("5574ca7b192e9dda0925e38a"), "val" : 11 }{ "_id" : ObjectId("5574ca7b192e9dda0925e38b"), "val" : 12 }{ "_id" : ObjectId("5574ca7b192e9dda0925e38c"), "val" : 13 }{ "_id" : ObjectId("5574ca7b192e9dda0925e38d"), "val" : 14 }{ "_id" : ObjectId("5574ca7b192e9dda0925e38e"), "val" : 15 }{ "_id" : ObjectId("5574ca7b192e9dda0925e38f"), "val" : 16 }{ "_id" : ObjectId("5574ca7b192e9dda0925e390"), "val" : 17 }{ "_id" : ObjectId("5574ca7b192e9dda0925e391"), "val" : 18 }{ "_id" : ObjectId("5574ca7b192e9dda0925e392"), "val" : 19 }Type "it" for more> db.pages.find().limit(5){ "_id" : ObjectId("5574ca7b192e9dda0925e37f"), "val" : 0 }{ "_id" : ObjectId("5574ca7b192e9dda0925e380"), "val" : 1 }{ "_id" : ObjectId("5574ca7b192e9dda0925e381"), "val" : 2 }{ "_id" : ObjectId("5574ca7b192e9dda0925e382"), "val" : 3 }{ "_id" : ObjectId("5574ca7b192e9dda0925e383"), "val" : 4 }可以发现，如果使用 limit方法的话会显示整个集合的所有文档。 指定了 limit 之后， 显示具体的条数，上文中，limit(5) 表示， 显示5条文档。
limit方法除外，还有一个 skip 方法，skip也是接受一个整型的参数，表示查询结果跳过多少个文档。
例如上面插入的30条记录中，我们要显示18-22条记录，那么就应该使用
db.pages.find().skip(18).limit(5)
> db.pages.find().skip(18).limit(5){ "_id" : ObjectId("5574ca7b192e9dda0925e391"), "val" : 18 }{ "_id" : ObjectId("5574ca7b192e9dda0925e392"), "val" : 19 }{ "_id" : ObjectId("5574ca7b192e9dda0925e393"), "val" : 20 }{ "_id" : ObjectId("5574ca7b192e9dda0925e394"), "val" : 21 }{ "_id" : ObjectId("5574ca7b192e9dda0925e395"), "val" : 22 }skip和limit的组合就能做到分页的功能了。但是如果数据量很大，理论上分页就会变得很慢了，比如有一亿条数据，要拿最后一页。那skip的数据量就很多很多了。这样就会变得比较慢。话说回来，有谁会看数据看到最后的几页？正常都是看前面几页数据，所以，skip和limit实现分页是可以接受的。
在mongodb中，如果要对查询结果排序，那么需要使用sort方法。sort方法接收一个文档参数。也就是{key:value}的形式。其中，key表示要排序的字段，value的可取值为 1 / -1 。1表示升序asc，-1表示降序desc。话不多说，直接上例子：
> db.pages.find().sort({val:-1}){ "_id" : ObjectId("5574ca7b192e9dda0925e39c"), "val" : 29 }{ "_id" : ObjectId("5574ca7b192e9dda0925e39b"), "val" : 28 }{ "_id" : ObjectId("5574ca7b192e9dda0925e39a"), "val" : 27 }{ "_id" : ObjectId("5574ca7b192e9dda0925e399"), "val" : 26 }{ "_id" : ObjectId("5574ca7b192e9dda0925e398"), "val" : 25 }{ "_id" : ObjectId("5574ca7b192e9dda0925e397"), "val" : 24 }{ "_id" : ObjectId("5574ca7b192e9dda0925e396"), "val" : 23 }{ "_id" : ObjectId("5574ca7b192e9dda0925e395"), "val" : 22 }{ "_id" : ObjectId("5574ca7b192e9dda0925e394"), "val" : 21 }{ "_id" : ObjectId("5574ca7b192e9dda0925e393"), "val" : 20 }{ "_id" : ObjectId("5574ca7b192e9dda0925e392"), "val" : 19 }{ "_id" : ObjectId("5574ca7b192e9dda0925e391"), "val" : 18 }{ "_id" : ObjectId("5574ca7b192e9dda0925e390"), "val" : 17 }{ "_id" : ObjectId("5574ca7b192e9dda0925e38f"), "val" : 16 }{ "_id" : ObjectId("5574ca7b192e9dda0925e38e"), "val" : 15 }{ "_id" : ObjectId("5574ca7b192e9dda0925e38d"), "val" : 14 }{ "_id" : ObjectId("5574ca7b192e9dda0925e38c"), "val" : 13 }{ "_id" : ObjectId("5574ca7b192e9dda0925e38b"), "val" : 12 }{ "_id" : ObjectId("5574ca7b192e9dda0925e38a"), "val" : 11 }{ "_id" : ObjectId("5574ca7b192e9dda0925e389"), "val" : 10 }Type "it" for more这个是对val这个key进行逆序排序，所以value取值为-1。 那value值为1的话，就变成升序了。
> db.pages.find().sort({val:1}){ "_id" : ObjectId("5574ca7b192e9dda0925e37f"), "val" : 0 }{ "_id" : ObjectId("5574ca7b192e9dda0925e380"), "val" : 1 }{ "_id" : ObjectId("5574ca7b192e9dda0925e381"), "val" : 2 }{ "_id" : ObjectId("5574ca7b192e9dda0925e382"), "val" : 3 }{ "_id" : ObjectId("5574ca7b192e9dda0925e383"), "val" : 4 }{ "_id" : ObjectId("5574ca7b192e9dda0925e384"), "val" : 5 }{ "_id" : ObjectId("5574ca7b192e9dda0925e385"), "val" : 6 }{ "_id" : ObjectId("5574ca7b192e9dda0925e386"), "val" : 7 }{ "_id" : ObjectId("5574ca7b192e9dda0925e387"), "val" : 8 }{ "_id" : ObjectId("5574ca7b192e9dda0925e388"), "val" : 9 }{ "_id" : ObjectId("5574ca7b192e9dda0925e389"), "val" : 10 }{ "_id" : ObjectId("5574ca7b192e9dda0925e38a"), "val" : 11 }{ "_id" : ObjectId("5574ca7b192e9dda0925e38b"), "val" : 12 }{ "_id" : ObjectId("5574ca7b192e9dda0925e38c"), "val" : 13 }{ "_id" : ObjectId("5574ca7b192e9dda0925e38d"), "val" : 14 }{ "_id" : ObjectId("5574ca7b192e9dda0925e38e"), "val" : 15 }{ "_id" : ObjectId("5574ca7b192e9dda0925e38f"), "val" : 16 }{ "_id" : ObjectId("5574ca7b192e9dda0925e390"), "val" : 17 }{ "_id" : ObjectId("5574ca7b192e9dda0925e391"), "val" : 18 }{ "_id" : ObjectId("5574ca7b192e9dda0925e392"), "val" : 19 }Type "it" for more>那如果要对多个值进行组合排序呢？ 就好比如对于我们最初的例子 users 集合。 要对users集合进行排序。其中 按照likes升序， 按照username降序。为了达到我们的效果，我们再往users集合里面插入两条数据
> db.users.insert({username:"mongodb",likes:999})> db.users.insert({username:"springMVC",likes:888})下面是运行结果，注意观察 likes为888的两个文档。发现username逆序排序了。至此，说明我们的sort实验成功了。
> db.users.find().sort({likes:1,username:-1}).pretty(){        "_id" : ObjectId("5574bdf3c705777157a515ac"),        "username" : "spring",        "url" : "webinglin.github.io",        "tags" : [                "spring",                "framework"        ],        "likes" : 777,        "author" : "linwenbin"}{        "_id" : ObjectId("5574cefa192e9dda0925e39e"),        "username" : "springMVC",        "likes" : 888}{        "_id" : ObjectId("5574bdd2c705777157a515ab"),        "username" : "redis",        "url" : "webinglin.github.io",        "tags" : [                "redis",                "database",                "nosql"        ],        "likes" : 888,        "author" : "linwenbin"}{        "_id" : ObjectId("5574cef5192e9dda0925e39d"),        "username" : "mongodb",        "likes" : 999}{        "_id" : ObjectId("5574bdabc705777157a515aa"),        "username" : "mongo",        "url" : "webinglin.github.io",        "tags" : [                "mongodb",                "database",                "nosql"        ],        "likes" : 999,        "author" : "linwenbin"}>
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