MongoDBCSC309 TA: Sukwon Oh

Review● SQL

○ “declarative” language for querying data○ tells what to find and not how to find

Review● RDBMS Characteristics

○ Easy to use○ Complicated to use it right○ Fixed schema○ Difficult to handle large amount of data

● Why?○ How the system works is hidden!

Review● NoSQL Philosophy (Not-Only SQL)

○ Simpler DB operations○ Complex query logic is moved to application code○ Relaxes ACID guarantees for better performance

Review● Types of NoSQL DB

○ Key-Value Store

○ Key-Data Structure Store

○ Key-Document Store

Why MongoDB?● To handle big amount of data● Better performance● Handle failures easily● Dynamic schema● Don’t need complex query operations● Your app is already using JSON● ...

Basics

Basics

Basics● Each DB has a set of collections● Each collection has a set of documents

○ Document looks like JSON format○ All documents have _id field with unique values

(primary key)● Each document contains a set of key-value

pairs

Sample Document{

“address”: {“building”: “1007”,“coord”: [-73.856077, 40.848447],“street”: “Morris Park Ave”,

...

How to use MongoDB?● First install MongoDB by following

instructions at https://www.mongodb.org/downloads

● Install node.js MongoDB driver (or any other language you want to use)○ npm install mongodb

Node.js Driver● MongoDB API that you can use from

express.js

● Supports CRUD operations○ CRUD - create, read, update, delete

Connecting to DBvar client = require(‘mongodb’).MongoClient;client.connect(url, function(err, db) {

console.log(‘connected!’);db.close(); // closes connection

});

MongoDB URI● Address of MongoDB instance you want to

connectmongodb://[username:password@]host[:port][/database][?options]● Example

○ ‘mongodb://localhost:27017/test’

Create● insertOne(), insertMany()

○ inserts 1 or multiple documents to a collection specified.

○ _id field is automatically generatedinsertOne Syntax:db.collection(‘collection’).insertOne(

{//document}, function(err, result){..});

CreateinsertMany Syntax:db.collection(‘collection’).insertMany(

[{//doc1}, {//doc2},...], function(err, result){..});

Read● Use find() to query DB

○ All queries are performed on a single collection○ Use filters to select only interesting documents

■ Similar to WHERE clause in SQL○ Returns cursor object, which you can use to iterate

over query results■ Use cursor.each() to look at individual document

Filter● Filters are in following format{<field1>: <value1>, <field2>: <value2>..}● Examples

○ {“borough”: “Manhattan”}○ {“addresses.zipcode”: “10075”}○ {“ratings”: [5,8,9]}○ {“ratings.0”: 5}

Examplevar cursor = db.collection(‘restaurants’).find({“borough”: “Manhattan”});cursor.each(function(err, doc) {

console.log(doc);});

Query Operators● Filter can be complex by using following

operators○ $or○ $gt○ $lt○ $elemMatch○ $eq○ ...

Query Operators{<field1>: {<operator1>: <value1>}}

● Examples○ {“grades.score”: {$gt: 30}}○ {ratings: {$elemMatch: {$gt: 5, $lt: 9}}}

Query Operators● Logical AND

○ Write multiple filters separated by commas

● Example○ {“cuisine”: “Italian”, “address.zipcode”: “10075”}

Query Operators● Logical OR

○ Use $or

● Examples○ {$or: [ {“cuisine”: “Italian”}, {“address.zipcode”:

“10075”} ] }

Sorting● Use .sort() method after .find()

○ Similar to ORDER BY in SQL● Accepts a document with keys to sort by and

values as 1 for ascending order and -1 or descending order

● Example○ {“borough”: 1, “address.zipcode”: -1}

Cursor methods● We have used each() but other methods are

available○ skip(# doc to skip)○ sort()○ next()○ toArray()○ map()

Update● updateOne(), updateMany(), replaceOne()● Accepts 3 paramters

○ a filter○ update value○ options

● Cannot update _id field!

Update Operators● $set to change a field value● $currentDate to update a field value to

current date● ...

UpdateupdateOne/updateMany Syntax:db.collection(‘collection’).updateOne/Many(

{//filter},{//update value},function(err, result) {..}

);

Exampledb.collection(‘restaurants’).updateMany(

{“address.zipcode”: “10016”, cuisine: “Other”},{

$set: {cuisine: “Category to be determined”},$currentDate: {“lastModified”: true}

},function(err, results) {

console.log(results);}

);

replaceOne● replaceOne() will only preserve field values

that are updated and throw away existing fields that are untouched!

● Same syntax as insertOne()

More on Update● By default, MongoDB does nothing when

none of documents are matched. However when upsert option is set to true, it will insert a new document if no matching documents are found.

Delete● deleteOne(), deleteMany()● Accepts a filter to choose which documents

to delete● Does not delete indexes or collections

○ Indexes are explained in next tutorial

DeleteSyntax:db.collection(‘collection’).deleteMany(

{//filter}, function(err, result) {..});

Exampledb.collection(‘restaurants’).deleteOne(

{“borough”: “Queens”},function(err, results) {

console.log(results);}

);

Drop● To completely remove a collection, consider

using drop()● Many be more efficient than deleteMany({})Syntax:db.collection(‘collection’).drop(function(err, results) {..});

Aggregation● Operates on multiple documents● Similar to the popular MapReduce paradigm● Perform stage-based aggregation

○ Example■ Stage 1: Filter out uninteresting documents■ Stage 2: Group documents by some key■ Stage 3: Count # of documents with same key

AggregationSyntax:db.collection.aggregate([<stage1>, <stage2>, ..])

Group Stage● Use $group to specify a stage{$group: {“_id”: “$key”, “field1”: {//accumulator}}

● Example○ {$group:{“_id”:”$borough”, “count”: {$sum: 1}}}

Accumulators● Only available in group stage and computes

values by combining documents with same key○ $sum○ $avg○ $max○ $min

Match Stage● Filters documents● Uses query syntax from before{$match: {//filter}}

Exampledb.collection(‘restaurants’).aggregate(

[{$match: {“borough”: “Queens”, “cuisine”: “Brazilian”}},{$group: {“_id”: “$address.zipcode”, “count”: {$sum: 1}}}

]).toArray(function(err, result) {

console.log(result);});