MongoDb join

In SQL Database, there are many ways to create the join between two or more tables like
Left Join
Right Join
Inner Join
Full Join
Self Join

But MongoDB is a NoSQL Database so It has one method to join between two or more collection through lookup().

Syntax:-

db.collectionName.aggregate([
    { $lookup:
       {
         from: 'otherCollection',
         localField: 'collection_fieldname',
         foreignField: 'otherCollection_fieldname',
         as: 'collectionDetails'
       }
     }
    ]).toArray();

Suppose, You have two collections First is employees collection which has 3 documents and every document has 5 key fields like _id, name, age, department and salary.

[
	{
		"_id" : ObjectId("5f26e736deec6e20ea057831"),
		"name" : "John",
		"age" : 35,
		"department" : "department A",
		"salary" : 200000
	},
	
	{
		"_id" : ObjectId("5f26e9dedeec6e20ea057833"),
		"name" : "Tony",
		"age" : 31,
		"department" : "department B",
		"salary" : 40000
	},
	
	{
		"_id" : ObjectId("5f26e9dedeec6e20ea057835"),
		"name" : "Andrew",
		"age" : 33,
		"department" : "department C",
		"salary" : 20000
	}
]

2) Second collection is department. which has 4 key fields like _id, name, floor and facility.

[
	{
		"_id" : ObjectId("5f27bfaddeec6e20ea057836"),
		"name" : "department A",
		"floor" : 5,
		"facility" : "standard"
	},
	{
		"_id" : ObjectId("5f27bfaddeec6e20ea057837"),
		"name" : "department B",
		"floor" : 4,
		"facility" : "good"
	},
	{
		"_id" : ObjectId("5f27bfaddeec6e20ea057838"),
		"name" : "department C",
		"floor" : 3,
		"facility" : "normal"
	}
]

Now you want to get the employees collection document with department details

db.employees.aggregate([
    { $lookup:
       {
         from: 'department',
         localField: 'department',
         foreignField: 'name',
         as: 'departmentDetails'
       }
     }
    ]).toArray();

Output:-