MongoDB

Prerequisites

Start the mongoDB server using the command mongod in the terminal
Start the mongoDB interactive shell using the command mongo on a separate terminal.

NOTE : Sometimes the mongod server is already running in the background in ubuntu. So first just try running the command mentioned in the second step i.e. mongo. If it doesn’t work then start the server and the interactive shell, one after another respectively.

After successfully entering the interactive shell try out the following stuff :

CREATE
READ
UPDATE
DELETE
Resources

CREATE

Creating a database

To create a database enter the command use db_name where db_name is the name of the database you want to create.
This command is also used to switch between databases
Eg:-
```
 use cars
```

List databases

To view the various databases created enter :-
```
 show dbs
```
Result:
```
 admin   0.000GB
 config  0.000GB
 local   0.000GB
 temp    0.000GB
```
NOTE : The cars db created in the earlier command isn’t visible as it has no data in it. Once a collection is created it will be visible on executing the above command.
To view the database you are currently using enter:-
```
 db
```

Creating collections

Collections are in mongoDB what tables are in SQL.
The only difference is that you don’t need to have a defined structure of the column names and the datatype those columns will have in mongoDB unlike SQL.
You can directly create a collection without specifying any details of the data that will be entered in it.
This can be done using the command db.createCollection('collection_name')
Eg:-
```
 db.createCollection('car_details')
```

Inserting documents

In mongoDB when you make an entry in a collection that entry is called a document i.e. a row in terms of SQL
You can insert a document in a collection using the command db.collection_name.insert(data)
Structure of the data in the example below :-
- brand
- model_name
- type
- engine
  - fuel
  - cc
  - bhp
- color
- safety
  - airbag (if airbags isn’t present then this field is omitted else its type(i.e. airbag type, for eg:- front/front and rear) is associated with the key)
  - sb_warn (seatbelt warning)

Eg:-

 db.car_details.insert(
     {
         'brand': 'chevrolet',
         'model_name': 'beat',
         'type': 'hatchback',
         'engine': {
             'fuel': 'petrol',
             'cc': 1000,
             'bhp': 100
         },
         'color': ['red', 'blue', 'green'],
         'safety': {
             'sb_warn': false
         },
         'price' : 400000
     }
 )

You can insert multiple entries in one go by using an array of objects i.e.
```
 db.collection_name.insert([{},{},....])
```
Copy and paste the contents of this file in your terminal.
- The file contains an array of objects containing the car_details.

READ

Fetching data from the documents is done using the find() function.
Eg:-
```
 db.car_details.find()
```
This returns all the documents in the car_details collection as no parameter was passed as to which filter(s) should be applied.

To get a readable format of the results use the pretty() function
Eg: -
```
  db.car_details.find().pretty()
```
This returns a properly formatted result of the find() output.

Filters

To get the document(s) based on some filters enter the key and the corresponding value as the first parameter to the find() function.
Eg:-

Here we want to get the details of the cars of type sedan

 db.car_details.find({ 'type' : 'sedan' }).pretty()

You could relate this type of filtering to the WHERE clause in SQL.

Comparison Operators

Comparison operators can be used to filter documents in monogDB using the following specifiers:-
- $gt = greater than
- $gte = greater than equal to
- $lt = less than
- $lte = less than equal to
- $ne = not equal to

Eg:- ($lte)

db.car_details.find({
    'price' : { '$lte' : 500000 }
}).pretty()

Logical Operators

Logical operators can be used to filter documents in mongoDB using the following specifications:-
- $or = OR operation
- $and = AND operation
Eg:- (AND)
```
 db.car_details.find({'$and' : [
     { 'type' : 'sedan' },
     { 'price' : { '$gte' : 500000 } }
 ]}).pretty()
```
Notice that the following command will also output the same result as the previous, this is because the default operation in case of multiple filters is an AND operation.

Eg:- (AND)(without the $and specifier)

 db.car_details.find({
      'type' : 'sedan' ,
      'price' : { '$gte' : 500000 }
 }).pretty()

Eg:- (OR)

 db.car_details.find({'$or': [
     { 'type' : 'sedan' },
     { 'price' : { '$gte' : 500000 } }
 ]}).pretty()

Functions

sort()
- Sort function can be used to sort the result of the query based on a key
- For ascending set the value of the key to 1
  and for descending set it to -1
- Eg:- SORT (desc)
```
  db.car_details.find().sort(
      {'price' : -1}
  ).pretty()
```
limit()
- To limit the no. of results of the query, send the no of results to be displayed as a parameter to the limit function
- Eg:-
```
  db.car_details.find().limit(3).pretty()
```
count()
- To just display the no of results that the query will output the count() function can be used
- Eg:-
```
  db.car_details.find().count()
```

Miscellaneous

$exists
- To check whether a key exists in the documents in the collections the $exists specifier
- Eg:-
```
   db.car_details.find({
       'safety.airbags': {'$exists' : true}
   }).pretty()
```
$regex
- Regular expressions can also be used to find entries having a pattern using the $regex specifier
- Eg:-
```
   db.car_details.find({
       'model_name' : { $regex: /^e/ }
   }).pretty()
```

UPDATE

There are 3 functions associated to updation of documents in mongoDB :-

updateOne()

This method is used to update the first matching document based on the filters provided

Eg:-

db.car_details.updateOne(
    { 'model_name' : 'cruze' },
    { '$set' : { 'price' : 1500000 } }
)

updateMany()
- This method is used to update all the matching documents based on the filters provided
- Eg:-
```
db.car_details.updateMany(
    { },
    { '$rename' : { 'body_type' : 'type' } }
)
```

replaceOne()

This method is used to update the first matching document based on the filters provided

Eg:-

db.car_details.replaceOne(
  { 'model_name' : 'jazz' },
  {
      'brand': 'honda',
      'model_name': 'jazz',
      'type': 'hatchback',
      'engine': {
          'fuel': 'diesel',
          'cc': 1150,
          'bhp': 115
      },
      'color': ['silver','orange','white'],
      'safety': {
          'airbags' : 'front',
          'sb_warn': true
      },
      'price' : 750000
  }
)

DELETE

Deleting documents

One or many documents can be deleted in mongoDb using the following commands:-
- deleteOne()
  - This method deletes the first document that matches the conditions specified
  - Eg:-
```
db.car_details.deleteOne(
       { 'model_name' : 'city' }
)
```
- deleteMany()
  - This method deletes the all the documents that match the conditions specified. If no condition is specified then all documents in the collection are deleted.
  - Eg:-
```
db.car_details.deleteMany(
       { 'body_type' : 'suv' }
)
```

Deleting collections

To delete a collection enter db.collection_name.drop()
Eg:-
```
db.car_details.drop()
```

Deleting databases

To delete the database you are currently working on, enter :-
```
db.dropDatabase()
```

Resources

NOTE : The official documentation is more than sufficient to get a hold of both basic and advanced concepts in mongoDB as it has an exhaustive no. of examples associated with every functionality. So, it is recommended to first have a look at the documentation before searching for a tutorial.

MongoDB

Prerequisites

Contents

CREATE

Creating a database

List databases

Creating collections

Inserting documents

READ

Filters

Comparison Operators

Logical Operators

Functions

Miscellaneous

UPDATE

DELETE

Deleting documents

Deleting collections

Deleting databases

Resources