Aggregation operators
In this section, we will learn how to use aggregation operators. Aggregation operators are divided into two categories. Within each stage, we use expression operators to compare and process values. Between different stages, we use aggregation stage operators to define the data that will get passed on from one stage to the next, as it is considered to be in the same format.
Aggregation pipeline stages
An aggregation pipeline is composed of different stages. These stages are declared in an array and executed sequentially, with the output of every stage being the input of the next one.
The last stage of the pipeline has to be either an $out operator or a $merge operator.
The $out stage has to be the final stage in an aggregation pipeline, outputting data to an output collection that will be completely erased and overwritten if it already exists. The $out operator cannot output to a sharded collection.
Additionally, the $merge stage has to...
