mongodb文档查询：

> db.centos_hr_study.find()

{ "_id" : ObjectId("5b61d2bf2d2a29b484b2a8b8"), "name" : "这是一个在centos中建立的用于学习的mongodb数据库" }

{ "_id" : ObjectId("5b61d55d2d2a29b484b2a8ba"), "name" : "张三", "age" : "30", "height" : "130" }

{ "_id" : ObjectId("5b61d5752d2a29b484b2a8bb"), "name" : "张4", "age" : "40" }

{ "_id" : ObjectId("5b61d7932d2a29b484b2a8be"), "name" : "赵5", "age" : "50", "height" : 150, "sex" : "不男不女" }

{ "_id" : ObjectId("5b61d79d2d2a29b484b2a8bf"), "name" : "李6", "age" : "60", "height" : 160, "sex" : "女" }

查找age<=40的，没查到：

> db.centos_hr_study.find({"age":{$lte:40}})

查找age<="40"的，有查到：

> db.centos_hr_study.find({"age":{$lte:"40"}})

{ "_id" : ObjectId("5b61d55d2d2a29b484b2a8ba"), "name" : "张三", "age" : "30", "height" : "130" }

{ "_id" : ObjectId("5b61d5752d2a29b484b2a8bb"), "name" : "张4", "age" : "40" }

查找age<="40" 且 height>="120"的，有查到：

> db.centos_hr_study.find({"age":{$lte:"40"},"height":{$gte:"120"}})

{ "_id" : ObjectId("5b61d55d2d2a29b484b2a8ba"), "name" : "张三", "age" : "30", "height" : "130" }

>