Aggregation Process in MongoDB

Let's first look at an aggregation process. We will not dire too deeply in details now, just high-level overview. Let's suppose at the beginning we have some documents in the collection. So entire collection is here. First we can perform Match operation like we did in "find()" or "update()" methods. This match query can produce subset of the documents. Then you can take this subset of the documents and perform group operation. As a result of the group operation you will get brand new documents. You can group matched documents based on certain condition. For example by age, by score, by state, by country and so on. And as a result of operation you will get new documents and each document will represent each group. For example each country, for example we have collection of persons. We can match persons that live for example just in USA and then group by state and in the results we will get documents, each document will represent certain state and each document will contain quantity of the persons that live in certain state. So aggregation is looks like a pipe. So we have large set of documents, then we make it smaller and smaller and at the end we will get some groups of those documents. Again this is a high level and later in the course we will dire in deeper details. Let's go on and next talk about aggregate() method and about its syntax. See in a second.