MongoDB (2)

Mongodb

PS a lot of MongoDB are not set a password, the default MongoDB is not set the password, halo

$type operator

The query type uses the
Gets the title value of the Col collection as a string type document

db.col.find(title: {$type: 2});

The limit and skip method limit queries the specified method

> db.ming.find().pretty();{ "_id" : ObjectId("5b5f6eb7d595bb04cbe14b96"), "ming" : "sdfrf" }{ "_id" : ObjectId("5b5f6f0ad595bb04cbe14b97"), "name" : "refgerfg" }{ "_id" : ObjectId("5b5f6f40d595bb04cbe14b98"), "x" : 222, "y" : 44444 }{ "_id" : ObjectId("5b5f6f50d595bb04cbe14b99"), "x" : 4444, "y" : 22222 }> db.ming.find().limit(2);{ "_id" : ObjectId("5b5f6eb7d595bb04cbe14b96"), "ming" : "sdfrf" }{ "_id" : ObjectId("5b5f6f0ad595bb04cbe14b97"), "name" : "refgerfg" }

Skip is skipping the first few data sorts

> db.ming.find({}, {ming:1, name:1}).sort({_id:-1});{ "_id" : ObjectId("5b5f6f50d595bb04cbe14b99") }{ "_id" : ObjectId("5b5f6f40d595bb04cbe14b98") }{ "_id" : ObjectId("5b5f6f0ad595bb04cbe14b97"), "name" : "refgerfg" }{ "_id" : ObjectId("5b5f6eb7d595bb04cbe14b96"), "ming" : "sdfrf" }> db.ming.find({}.{ming:1, name:1}).sort({id:1});2018-07-31T19:49:11.007+0800 E QUERY    [js] SyntaxError: missing name after . operator @(shell):1:16> db.ming.find({}, {ming:1, name:1, x:1, y:1}).sort({ _id:1});{ "_id" : ObjectId("5b5f6eb7d595bb04cbe14b96"), "ming" : "sdfrf" }{ "_id" : ObjectId("5b5f6f0ad595bb04cbe14b97"), "name" : "refgerfg" }{ "_id" : ObjectId("5b5f6f40d595bb04cbe14b98"), "x" : 222, "y" : 44444 }{ "_id" : ObjectId("5b5f6f50d595bb04cbe14b99"), "x" : 4444, "y" : 22222 }>

Index

> db.ming.createIndex({ming:1});{        "createdCollectionAutomatically" : false,        "numIndexesBefore" : 1,        "numIndexesAfter" : 2,        "ok" : 1}>

The Ming field creates a sequence in ascending order

Polymerization

Count () similar to SQL

db.ming.aggregate(        [            (                $groupp: {                    _id : "$by-user",                    num_tutorial: {                        $sum: 1                    }                }            )        ]    )

Groups are grouped by _id, and the grouped documents are counted, that is, the sum of the grouped documents.

Pipeline

> db.ming.aggregate(... {... $project : {... _id:0,... x:1,... y:1... }... }... ){  }{  }{ "x" : 222, "y" : 44444 }{ "x" : 4444, "y" : 22222 }

Once a value is processed, it is passed to the next processing
That is, when the value passes through the pipeline, it becomes the appropriate format

Get documents randomly

db.ming.aggregate(        [            {                $sample: {                    size:1                }            }        ]    )

Use aggregation to achieve the purpose with $sample, where size is the number of randomly fetched bars

SQL Random fetch two records

select * from dataorder by rand()limit 2

mysql> select * from c order by rand() limit 2;+--------+------------------------+-------+| cno    | cn                     | hourc |+--------+------------------------+-------+| 050192 | 专业英语               | 32    || 090112 |  | 48    |+--------+------------------------+-------+2 rows in set (0.10 sec)mysql> select * from c order by rand() limit 2;+--------+----------------------+-------+| cno    | cn                   | hourc |+--------+----------------------+-------+| 050198 | 计算机网络安全与应用 | 32    || 050150 | 计算机组装与维护     | 36    |+--------+----------------------+-------+2 rows in set (0.03 sec)mysql>

The rest of the copy, can not see anymore.

MongoDB (2)