What is the difference between find() and aggregate() in MongoDB?

find() is optimized for simple queries — filtering, projecting, sorting, and limiting documents from a collection. It's direct and efficient for most read operations. Best for: fetching documents with simple filters and field selection. Performance: uses indexes directly, low overhead. Syntax: db.collection.find(query, projection).sort().limit(). aggregate() is a powerful pipeline framework for complex data processing — transformations, grouping, joining, computing, and reshaping data. Best for: GROUP BY operations, JOINs ($lookup), complex transformations, computed fields, multi-stage data pipelines, analytics. Performance: more overhead than find(), but can be optimized with early $match and $project stages. When to use each: find(): simple lookups, pagination, filtering with projections — it's faster and simpler; aggregate(): any grouping, joining, or transformation — whenever you need more than find() can do. Can find() do everything aggregate() can? No — find() can't do GROUP BY, JOINs, computed aggregations, $unwind of arrays, or complex transformations. Aggregation with $match only: db.users.aggregate([{ $match: { age: { $gte: 18 } } }]) is equivalent to find() but slightly less efficient — use find() for simple filters. Using aggregate() for pagination: [{ $match: filter }, { $sort: sort }, { $skip: skip }, { $limit: limit }, { $project: projection }] — allows combining count and data in one pass with $facet.