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What is the difference between SQL and NoSQL databases?

SQLNoSQLDatabaseMongoDBMySQL

Question

Explain the differences between SQL and NoSQL databases. When would you choose one over the other?

Interview Answer

SQL databases are best for structured relational data and strong transactional consistency. NoSQL databases are better for flexible schema and horizontal scalability. The right choice depends on access patterns and consistency requirements.

Explanation

SQL excels where relationships, JOIN-heavy queries, and strict consistency matter. NoSQL shines for schema flexibility, high write throughput, and distributed scale. Many systems use both, combining transactional and analytical workloads.

Key Points

  • SQL is schema-first and strong for relational consistency
  • NoSQL is schema-flexible and scales horizontally with ease
  • Choose based on workload: transactional integrity vs flexible high-scale reads/writes
  • Hybrid architecture is common in real systems

Common Mistakes

  • Claiming NoSQL cannot support transactions at all
  • Choosing NoSQL only for trend value without workload analysis
  • Ignoring query patterns before selecting the database type

Likely Follow-Up Questions

  • How would you model an e-commerce order system in SQL vs NoSQL?
  • When would you use a document store and a relational DB together?
  • How do indexing and partitioning differ between SQL and NoSQL systems?

Interview Timing

Target speaking time: about 4 minutes.

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