MongoDB中使用find来进行查询，返回一个集合中的文档的子集；
返回文档集合blog中的所有文档：
> db.post.find()
{ "_id" : ObjectId("54a530c3ff0df3732bac1681"), "id" : 2, "name" : "joe", "age" : 30, "sex" : 1, "school" : "marry" }
{ "_id" : ObjectId("54a530c3ff0df3732bac1680"), "id" : 1, "name" : "joe", "age" : 30, "comments" : [ "test2", "test9", "test5" ], "sex" : 1, "school" : "marry" }
>返回指定的文档：
> db.post.find({"id":1})
{ "_id" : ObjectId("54a530c3ff0df3732bac1680"), "id" : 1, "name" : "joe", "age" : 30, "comments" : [ "test2", "test9", "test5" ], "sex" : 1, "school" : "marry" }
>返回指定的键值：
> db.post.find({},{"id":1,"age":1})
{ "_id" : ObjectId("54a530c3ff0df3732bac1681"), "id" : 2, "age" : 30 }
{ "_id" : ObjectId("54a530c3ff0df3732bac1680"), "id" : 1, "age" : 30 }
>默认总是会返回“_id”键，使用下面的方法可以不返回“ _id”键：
> db.post.find({},{"id":1,"age":1,"_id":0})
{ "id" : 2, "age" : 30 }
{ "id" : 1, "age" : 30 }
>
可以将多个条件组合在一起，例如查询名字为“Joe”且ID为1的文档：
> db.post.find({"name":"joe"})
{ "_id" : ObjectId("54a530c3ff0df3732bac1681"), "id" : 2, "name" : "joe", "age" : 30, "sex" : 1, "school" : "marry" }
{ "_id" : ObjectId("54a530c3ff0df3732bac1680"), "id" : 1, "name" : "joe", "age" : 30, "comments" : [ "test2", "test9", "test5" ], "sex" : 1, "school" : "marry" }
> db.post.find({"name":"joe","id":1})
{ "_id" : ObjectId("54a530c3ff0df3732bac1680"), "id" : 1, "name" : "joe", "age" : 30, "comments" : [ "test2", "test9", "test5" ], "sex" : 1, "school" : "marry" }
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