SQL vs NoSQL
maang.io System Design Series
"SQL vs NoSQL" is the most common — and most misunderstood — database question in interviews. This deep dive replaces the false binary with a framework: relational vs the four NoSQL families (key-value, document, wide-column, graph), ACID vs BASE, rigid vs flexible schema, and vertical vs horizontal scaling. The answer is almost always "it depends — here's on what."
This deep dive comes in three parts:
- Foundations & Core Concepts — the families, ACID vs BASE, and the scaling models.
- Internals — schema-on-read, join-vs-embed, and polyglot persistence, with worked examples.
- Interview — Staff/Principal Q&A: picking a store and defending the trade-offs.
SQL vs NoSQL — Part 3: System Design & Interview Mastery
maang.io System Design Series
What is this?
You now know both back rooms — the B+tree and the LSM-tree, joins and quorums, WAL and MVCC. This chapter is where that knowledge earns the offer. Here's the thing interviewers know that juniors don't: "SQL or NoSQL?" is almost never the real question. The real question is "can you decompose a system into the few stores whose shapes fit its parts, and defend every trade when I push back?" Nobody senior picks one database for a whole company. They run the central library and the specialized branches, and they can tell you exactly why each one is there.
So we'll do two things: see where each store lands in real designs (with a full polyglot architecture), then run a mock interview — for each question, what they're really asking, a model answer, and the senior line that signals you've actually shipped this.
The one-line idea: interviewers use SQL-vs-NoSQL to test whether you default to the boring correct choice, justify each departure from it with a workload shape, and stay honest about the costs — no joins, stale reads, denormalization tax — rather than pattern-matching on "NoSQL scales."
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