MongoDB vs MySQL

Problem What are the differences between MongoDB and MySQL, and when would you choose each?

Be ready to discuss

• Data model: MySQL's fixed schema of tables/rows/columns with normalization and foreign keys versus MongoDB's flexible, schema-less JSON-like BSON documents that embed related data as one aggregate.
• Joins versus embedding: MySQL joins across normalized tables; MongoDB has $lookup but denormalization/embedding is idiomatic, trading write-time duplication for single-round-trip reads.
• Transactions and consistency: MySQL/InnoDB's mature ACID guarantees and isolation levels versus MongoDB's document-level atomicity, with multi-document transactions available but costlier and less idiomatic.
• Scaling: MySQL scales vertically plus read replicas, with sharding bolted on; MongoDB ships with native sharding and replica sets, with tunable read/write concerns trading consistency for latency and availability.
• Indexing and querying: B-tree indexes and a cost-based SQL optimizer versus MongoDB's index types on document fields and array elements, and the aggregation pipeline in place of SQL.
• Choosing: MySQL when relational integrity, complex joins, and transactional correctness matter (ledgers, orders); MongoDB when schemas evolve fast, the access pattern is aggregate-shaped, or write throughput and horizontal scale dominate (catalogs, event/activity data).
• Worth noting: MySQL has a JSON column type and MongoDB has transactions, so the lines have blurred — the honest answer is about default fit, not capability checklists.