In RDBMS, we have different type of joins to join the tables and get the required data from joined tables. As MongoDB is a NO Sql database, MongoDB will not support those type of joins. But we can implement LEFT OUTER JOIN using $lookup function in MongoDB. This function is there in MongoDB 3.2 version. To join two collections we need to have a common field in both the collections. $lookup works with aggregate function only. Syntax: db.collection1.aggregate([ { $lookup: { from:"collection2", localField:"common field name from collection1", foreignField:"common field name from collection2", as:"Alias name for collection2" } } ]) Here in above syntax, collection1 -- Collection name which is acts like parent table. collection2 -- Collection name which is acts like child table and this will be joined with collection1. "common field name from collection1" -- Join column
