MongoDB is a NoSQL database that is one of the most customary documents accessible and stores both as a completely controlled cloud service and also for deploying on self-managed infrastructure. Since MongoDB is not an RDBMS type, but still, a user can execute a left outer join by means of the $lookup stage. This stage $lookup allows you to state which collection a user needs to join with the present collection and also which fields that must match. Show In MongoDB it is quite difficult to relate data from one collection to another excluding when basic script query functions are used. Therefore, to resolve this the JOIN concept is introduced which facilitates the data relation. MongoDB JOIN operation with two collections is performed by the use of $lookup operator developed having version 3.2. Syntax: The key concept behind the MongoDB JOIN operation is to achieve a correlation between one collection data to another one. This is done by using the $lookup operator so, the basic syntax for this to join two collections can be defined as follows: Start Your Free Data Science Course Hadoop, Data Science, Statistics & others
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In the case of the SQL concept, the result of a JOIN operation is always a different row that associates all fields present from the parent and foreign tables. But in MongoDB, the case is variant where the output documents are supplemented as an array of native collection documents. How to join two collections in MongoDB?For joining MongoDB collections, we require to implement the operator as $lookup, which is defined as a stage executing a left outer join with other collection and also aids to filter data info from joined documents.
We can see in the above query that the initial parameter named ‘from’ states the collection that is to be joined with the present one, on the other side the ‘localField’ parameter states the key prevailing in the present collection that is to be matched with the foreign key in another collection by means of ‘foreignField’ parameter. Here, Student_id from Students and stud_id from Grades are matched as well as joined for retrieving the data. Also, the end parameter ‘as’ is added as a code-named to the data. It must be noted that here in every record of result from lookup, also the data from the joined table appears inside an array as the primary element, every one row from the output data of the query above that will display like,
Here, in the grades array, the initial element will be the data that the query joined. ExampleLet us illustrate few examples to show the joining of two collections in MongoDB explained as follows: Students:
Marks:
We can fetch the students’ marks with corresponding grades by means of the $lookup operator using the JOIN methodology defined below:
On execution, the output will provide the following results,
The structure of the written query may decide the performance duration. Like for instance, when there are several documents in one collection over the other, then aggregation is needed from one collection with smaller documents followed by lookup in the one having various documents. This type of lookup performed for the selected field from the smaller documents collection is rather finest and proceeds smaller time than executing numerous lookups for a selected field present in the collection having various documents. Thus, it is sensible to place the smaller collection initially. But the order of the databases does not trouble in case of a relational database as maximum of the SQL interpreters includes optimizers that provide access to additional information to select which one must be the primary one. In this MongoDB, to simplify the JOIN operation we may require to implement an index. Since all MongoDB documents include an _id key that for a relational DBM is measured as the primary key. An index delivers a better chance of minimizing the quantity of data which requires to be retrieved besides associating the operation when applied in the $lookup foreign key. ConclusionJOIN operator is one of the basic distinctive features available between SQL database and NoSQL database. In SQL databases, a user can execute a JOIN operation between two tables inside identical or different databases. While the case in MongoDB for JOIN is varied, MongoDB lets JOIN operations perform between two collections implemented in a similar database. Recommended ArticlesThis is a guide to MongoDB Join Two Collections. Here we discuss definition, How to join two collections in MongoDB? along with examples respectively. You may also have a look at the following articles to learn more – How do I join multiple collections with $lookup MongoDB?MongoDb Lookup how to join multiple collections. apply $lookup before $group (used in case of high-cardinality). apply $lookup after $group (used in case of low-cardinality for more efficiency). Can a MongoDB have multiple collections?Yes, you can have multiple collections within a database in MongoDB. Collections can be thought of as analogous to tables in relational databases.
How to get data from 2 different collections of MongoDB using node js?After having a model, we can use method find() on the model of a particular collection to get documents of the collection. <query>: It is optional. It specifies a selection filter that is used to filter documents using various MongoDB query operators. If not passed, all the documents are returned.
How do I join a collection 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.
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