Software developer interview: how do you decide between SQL and NoSQL for a new feature?

Model answer

Show that you decide based on the problem, not fashion. Start with the principle: choose the data store that fits your data's shape and access patterns. Favour a relational SQL database when your data is well-structured with clear relationships, when you need strong consistency and transactions — for example anything touching money or bookings — and when you will run complex queries and joins. Favour a NoSQL store when you need flexible or evolving schemas, very high write throughput, or horizontal scale for large volumes, and when your access is mostly by a known key rather than complex joins; document, key-value, and wide-column stores each suit different shapes. Give a concrete example for the feature in question and reason aloud about consistency, query needs, and scale. Show maturity by noting it is not either-or — many systems use both, a relational core plus a cache or a document store for specific needs. Avoid dogmatic statements like "NoSQL is always faster". The interviewer wants a trade-off-driven decision tied to real requirements, so end by restating which you would pick for this feature and why.

This is a guide to structure and approach, not a script to memorise — say it in your own words.

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