Q1. How does MongoDB differ from a relational database?
MongoDB is a document database. Data is stored as flexible BSON documents inside collections instead of rows in tables with a fixed schema. Each document can have its own...
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Q2. When would you embed versus reference documents in MongoDB?
Embed when the child data is always read with the parent, is bounded in size, and belongs to that parent. Reference when the child is large, unbounded, or shared across m...
Q3. How do indexes work in MongoDB and what is a compound index?
An index is a B-tree structure storing a sorted subset of a collection's fields, letting the planner find documents without scanning everything. Without an index a query ...
Q4. Write an aggregation pipeline that returns the top three products by revenue.
Pipelines pass documents through stages, transforming them step by step. ```js db.orders.aggregate([ { $match: { status: "paid" } }, { $unwind: "$items" }, { $group: { _i...
Q5. What is the $lookup stage and what are its trade-offs?
$lookup performs a left outer join against another collection in the same database. The basic form matches localField against foreignField and returns matches in an "as" ...
Q6. How does sharding work in MongoDB?
Sharding horizontally partitions a collection across mongos routers and shards. You choose a shard key, and MongoDB splits the key space into chunks distributed and balan...
Q7. What are replica sets and how do they provide high availability?
A replica set is a group of nodes holding the same data: one primary and two or more secondaries. All writes go to the primary. Secondaries replicate the oplog and can se...
Q8. Explain write concern and read concern in MongoDB.
Write concern controls how many nodes must acknowledge a write before it counts as successful. w:1 means the primary acknowledged it; w:"majority" means a majority of rep...
Q9. How do you model a one-to-many relationship in MongoDB?
The answer depends on the size of the "many" side and the access pattern. One-to-few: embed an array of subdocuments. A user with a handful of addresses embeds them, so o...
Q10. When should you prefer the aggregation framework over find?
Use find for simple filtering, projection, sorting and pagination on a single collection. It is lightweight and uses indexes efficiently. Use aggregate when you must tran...