其中的字段代表,需要按哪个字段分组.变量表示这一个分组中会使用的变量,并且给一个初始值.可以在后面的$reduce函数中使用.$reduce的两个参数,分别代表当前的文档和上个文档执行完函数后的结果.如下我们按年龄分组,同级不同年龄的用户的多少:

[ html]

> db.user.find()

{ "_id" : ObjectId("5198c286c686eb50e2c843b2"), "name" : "user0", "age" : 0 }

{ "_id" : ObjectId("5198c286c686eb50e2c843b3"), "name" : "user1", "age" : 1 }

{ "_id" : ObjectId("5198c286c686eb50e2c843b4"), "name" : "user2", "age" : 2 }

{ "_id" : ObjectId("5198c286c686eb50e2c843b5"), "name" : "user3", "age" : 1 }

{ "_id" : ObjectId("5198c286c686eb50e2c843b6"), "name" : "user4", "age" : 1 }

{ "_id" : ObjectId("5198c286c686eb50e2c843b7"), "name" : "user5", "age" : 2 }

> db.user.group({key:{age:1},initial:{count:0},$reduce:function(doc,prev){prev.count++}})

[

{

"age" : 0,

"count" : 1

},

{

"age" : 1,

"count" : 3

},

{

"age" : 2,

"count" : 2

}

]

group更加详细的介绍请看http://www.2cto.com/database/201305/212159.html这篇文章

distinct

去除查询结果中的重复数据,对原有数据不会产生影响,返回的结果是一个数组.

[html]

> db.user.find()

{ "_id" : ObjectId("5198c286c686eb50e2c843b2"), "name" : "user0", "age" : 0 }

{ "_id" : ObjectId("5198c286c686eb50e2c843b3"), "name" : "user1", "age" : 1 }

{ "_id" : ObjectId("5198c286c686eb50e2c843b4"), "name" : "user2", "age" : 2 }

{ "_id" : ObjectId("5198c286c686eb50e2c843b5"), "name" : "user3", "age" : 1 }

{ "_id" : ObjectId("5198c286c686eb50e2c843b6"), "name" : "user4", "age" : 1 }

{ "_id" : ObjectId("5198c286c686eb50e2c843b7"), "name" : "user5", "age" : 2 }

> db.user.distinct("age")

[ 0, 1, 2 ]

分页查询

在mongodb中实现分页比较简单,需要使用到skip 和limit方法.skip表示跳过前面的几个文档,limit表示显示几个文档.

[html]

> db.user.find()

{ "_id" : ObjectId("5198c286c686eb50e2c843b2"), "name" : "user0", "age" : 0 }

{ "_id" : ObjectId("5198c286c686eb50e2c843b3"), "name" : "user1", "age" : 1 }

{ "_id" : ObjectId("5198c286c686eb50e2c843b4"), "name" : "user2", "age" : 2 }

{ "_id" : ObjectId("5198c286c686eb50e2c843b5"), "name" : "user3", "age" : 1 }

{ "_id" : ObjectId("5198c286c686eb50e2c843b6"), "name" : "user4", "age" : 1 }

{ "_id" : ObjectId("5198c286c686eb50e2c843b7"), "name" : "user5", "age" : 2 }

> db.user.find().skip(2).limit(3) -->跳过前两个文档查询后面的三个文档,经过测试这两个方法的使用顺序没有影响

{ "_id" : ObjectId("5198c286c686eb50e2c843b4"), "name" : "user2", "age" : 2 }

{ "_id" : ObjectId("5198c286c686eb50e2c843b5"), "name" : "user3", "age" : 1 }

{ "_id" : ObjectId("5198c286c686eb50e2c843b6"), "name" : "user4", "age" : 1 }

> db.user.find().limit(3).skip(2)

{ "_id" : ObjectId("5198c286c686eb50e2c843b4"), "name" : "user2", "age" : 2 }

{ "_id" : ObjectId("5198c286c686eb50e2c843b5"), "name" : "user3", "age" : 1 }

{ "_id" : ObjectId("5198c286c686eb50e2c843b6"), "name" : "user4", "age" : 1 }