Why joins are not used in mongodb?
MongoDB Joins are different from how Joins work on SQL. MongoDB is an open-source NoSQL Database used to store high volumes of data. In traditional databases, we use tables and rows but MongoDB uses Collections and Documents. Documents use key-value pairs which is the basic unit in MongoDB. Collections are a set of documents and functions which is the equivalent of tables in Relational Databases. Show This blog talks about how to use MongoDB Joins, the key types of Joins and Lookup commands. Table of Contents
What is MongoDB?MongoDB is an open-source NoSQL Database used to store high volumes of data and work with it efficiently. It is categorized as NoSQL because the storage and retrieval of data are not in the form of tables and rows. It is in the form of Documents and Collections. The MongoDB Database was initially released in February 2009 and is managed by MongoDB.inc. It also supports all the popular languages like C, C++, C#, DotNet, Java, Node.js, Ruby etc. There are a lot of companies that use MongoDB like Facebook, Nokia, Google etc. to store their large amounts of data. Although you might think that it is impossible to perform MongoDB Joins because it is a Non Relational Database, there are a few ways to do that. These are discussed in detail later in the blog. Key Features of MongoDB
Key Components of MongoDB ArchitectureImage Source
Here’s an example of Key-value Pairs:
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GET STARTED WITH HEVO FOR FREE What is MongoDB Lookup Aggregation?One of the major differences between SQL and NoSQL Databases is when it comes to the JOIN clause. In Relational Databases, the JOIN clause lets you combine two or more rows from tables using a common field. But in MongoDB, databases are designed to store denormalized data. Fortunately, MongoDB Joins can be performed in MongoDB 3.2 as it introduces a new Lookup operation that can perform Join operations on Collections. DefinitionMongoDB Joins are performed by Lookup. It performs a Left Outer Join to two or more Collections. But Lookup is only permitted in Aggregate operations. This is like a pipeline that performs query, filter and group operations. The output of one is used as input for the next operation. MongoDB Joins SyntaxThe following section talks about the syntax of different conditional clauses for MongoDB Joins:
MongoDB Joins: Equality Match with a Single Join ConditionThis clause performs an equality match between fields from the input documents and the joined collection. Syntax:
This would correspond to the following SQL code:
Examples:
MongoDB Joins: Conditions and Subqueries on Joined CollectionsMongoDB 3.6 has added support to:
To perform other join operations other than Equality Join, you can use this Lookup syntax:
This is equivalent to to the SQL code below:
Examples:
MongoDB Joins: Correlated SubQueries using Concise SyntaxFrom MongoDB 5.0 onwards, you can use a concise syntax for correlated subqueries. The following syntax removes the requirement for an equality match on the local and foreign fields.
The SQL code below is equivalent to the above syntax:
Example:
MongoDB Join RestrictionsThere are a few restrictions or limitations while using MongoDB Joins:
MongoDB Join ExamplesHere are a few examples of the different types of MongoDB Joins we have seen above:
Single Equality JoinThere are two Collections “orders” and “inventory”. Below you can see the code snippet:
The following Aggregation operations perform Join operation on both these Collections and Join them using fields “item” from “orders” and “sku” from “inventory”. The code snippet is below:
The result is as follows:
Using Lookup on an ArrayYou can match the array elements with a foreign field. For example, there are two Collections “classes” and “members” with the following Documents:
The following operation joins “members” and “classes” on members field to name field:
The output will be as follows:
Multiple Joins and Correlated SubQueryCreate a Collection “order’ and “warehouses” as follows:
You can use a Correlated SubQuery that performs join on “orders.item” and “warehouse.stock_item” fields as follows:
The output for the aforementioned code snippet would be:
Concise Correlated SubQueryCreate Collections “restaurants” and “orders” with the following Documents:
Join the above two Collections on orders.restaurant_name and restaurants.name and perform an $in array match between orders.drink and beverages as follows:
The output would be:
ConclusionThis blog discusses the features of MongoDB Joins and the types that we can perform on the data. MongoDB $lookup is very useful and powerful but it requires complex aggregate queries. Extracting complex data from a diverse set of data sources to carry out an insightful analysis can be challenging, and this is where Hevo saves the day! Hevo offers a faster way to move data from Databases or SaaS applications like MongoDB into your Data Warehouse to be visualized in a BI tool. Hevo is fully automated and hence does not require you to code. GET STARTED WITH HEVO FOR FREE If you are using MongoDB as a source of data and searching for a no-fuss alternative to Manual Data Integration, then Hevo can effortlessly automate this for you. Hevo, with its strong integration with 100+ sources & BI tools (Including 40+ Free Sources), allows you to not only export & load data but also transform & enrich your data & make it analysis-ready in a jiffy.
Want to take Hevo for a ride? SIGN UP for a 14-day free trial and simplify your Data Integration process. Do check out the pricing details to understand which plan fulfills all your business needs. Can we use joins in MongoDB?Join Collections
MongoDB is not a relational database, but you can perform a left outer join by using the $lookup stage. The $lookup stage lets you specify which collection you want to join with the current collection, and which fields that should match.
Why joins are not supported in NoSQL?Natively, unfortunately, is not possible to perform a Join into a NoSQL database. This is actually one of the biggest differences between SQL and NoSQL DBs. As @kaleb said, you would have to do multiple selections and then join the needed information "manually".
Can joins be used in NoSQL?Joins are predominantly used in an Oracle NoSQL Database when a user is trying to extract data from tables that have a hierarchical relationship between them.
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