Pipelines, Stages and Operators

MongoDB aggregation pipelines are composed of sequential stages that process and transform documents, where each stage performs a specific operation using various operators before passing results to the next stage. Stages like $match (filtering), $group (grouping and aggregating), $project (field selection and transformation), $sort (ordering), $lookup (joins), and $unwind (array expansion) can be combined in any order to create complex data processing workflows. Operators within these stages include arithmetic operators ($add, $multiply), comparison operators ($eq, $gt), array operators ($push, $addToSet), date operators ($dateToString, $year), and conditional operators ($cond, $ifNull), providing a powerful and flexible framework for data analysis, reporting, and ETL operations directly within the database.