MongoDB Learning Notes and Useful Commands

July 20, 2024 NoSql Database MongoDB Backend Development

MongoDB is one of the typical NoSQL databases. It’s a document databased where data is stored in the form of json-type document. Understanding MongoDB is crucial for back-end development, particularly for agile development.

Useful Commands

A. Search Commands

  1. start mongosh
  2. show databases
  3. show collections
  4. switch to the database
  5. find all datas in the database
  6. delete collection
  7. insert data
  8. get the count of the data
  9. find data in page n ( skip((n-1)*pageSize)) (Mongodb can show data by pages)
  10. find data by AND conditions
  11. find data by OR conditions
  12. find the first data
  13. find data with age greater than 22
  14. find data with age greater than and equals to 22
mongosh

show dbs

show collections

use databaseName

db.collectionName.find()

db.collectionName.drop()

db.collectionName.insert({"username": "Tom", "age": 18})

db.collectionName.find().count()

db.collectionName.find().skip((n-1) * pageSize).limit(pageSize)

db.collectionName.find({"username": "Tom", "age": 18})

db.collectionName.find({$or:[{"username":"Tom", "username":Mary}]})

db.collectionName.findOne()

db.collectionName.find({age:{$gt:22}})

db.collectionName.find({age:{$gte:22}})

B. Update Commands

Notes: don’t forget $set when updating data.

  1. update name from “Tom” to “Jack”
  2. add sex in one specified data(name=Jack, age=18)
  3. update multi data at one time
  4. increase age by 1
db.collectionName.update({"name": "Tom"}, {$set:{"name": "Jack"}})

db.collectionName.update({"name":"Jack", "age": 18}, {$set:{"sex": "male"}})

db.collectionName.update({"age": 18}, {$set:{"sex": "female"}}, {multi:true})

db.collectionName.update({"name": "Tom"}, {inc:{age: 1}}, false, true)

C. Delete Commands

  1. delete one specified data
  2. delete multiple data (age > 18)
  3. delete just one data which satisfied the condition
db.collectionName.remove({"name": "Tom"}})

db.collectionName.remove({"age": {$gt: 18}})

db.collectionName.remove({"age": {$gt: 18}}, {justOne:true})

D. Advanced MongoDB

Setting an index can make the search speed faster.

  1. set an index
  2. get the indexes
  3. set multiple indexes
  4. set unique index (A unique index means that the value cannot be repeated.) ```zsh db.collectionName.ensureIndex({“username”: 1})

db.collectionName.getIndexes()

db.collectionName.dropIndex({“username”: 1})

db.collectionName.ensureIndex({“username”: 1}, {“age”: 1})

db.collectionName.ensureIndex({“age”: 1}, {“unique”: true})


#### E. The relationships of Databases
##### 1. One-to-one
Like National ID to student ID.

##### 2. One-to-many
Like order ID to order items. In one order, there can be many items.


##### 3. Many-to-many
Like products to users. One product can be collected by multiple users. 
One user can collect multiple products. 

In many-to-many situation, we ofter use intermediate table to mark the correspondence.

#### F. Aggregation Pipeline
Aggregation pipeline in MongoDB is used to transfer and group documents.

Usage: Table Association Query and Data Statistics

##### 1. $project
$project is used to edit the structure of the documents. It's useful to rename, add and delete the data.

```zsh
db.collectionName.aggregate([
                    {$project: {trade_no: 1, all_price: 1}}
])
2. $match

$match is used to filter the documents.

db.collectionName.aggregate([
                    {$project: {trade_no: 1, all_price: 1}},
                    {$match: {"all_price": {$gte: 90}}}
])
3. $group

$group is used to filter the documents.

In the following example, data is grouped by order_id and then sum up by the num.

db.collectionName.aggregate([
                    {$group: {$_id: "order_id", total: {$sum: "$num"}}},
])
4. $sort

$sort the documents in the collection. {$sort:{“all_price”: -1}}

db.collectionName.aggregate([
                    {$project: {trade_no: 1, all_price: 1}},
                    {$match: {"all_price": {$gte: 90}}},
                    {$sort:{"all_price": -1}}
])
4. $limit

$limit stands for only returning one data. Only showing one data.

db.collectionName.aggregate([
                    {$project: {trade_no: 1, all_price: 1}},
                    {$match: {"all_price": {$gte: 90}}},
                    {$sort:{"all_price": -1}},
                    {$limit: 1}
])
5. $skip

$skip stands for skip data. Skip one data:

db.collectionName.aggregate([
                    {$project: {trade_no: 1, all_price: 1}},
                    {$match: {"all_price": {$gte: 90}}},
                    {$sort:{"all_price": -1}},
                    {$skip: 1}
])
6. $lookup

$lookup can connect two tables together.

db.collectionName.aggregate([
                    {$lookup:{
                      from:"order_item",
                      localField:"order_id",
                      foreignField: "order_id",
                      as:"items"
                    }},
                    {$project: {trade_no: 1, all_price: 1}},
                    {$match: {"all_price": {$gte: 90}}},
                    {$sort:{"all_price": -1}},
                    {$skip: 1}
])