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Databases

SQL & MongoDB interview questions for backend roles.

Database questions separate people who can write a query from people who can keep it fast and correct under load. Expect indexing, transactions and modelling trade-offs, not just SELECT statements.

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Nagendra Kumar

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Readiness assessment

Know exactly where you stand in Databases & Data Modelling.

  • Overall readiness score out of 100
  • Skill-wise breakdown across 7 areas
  • Your gaps, ranked by priority
  • An action plan and simple explanations for every miss

₹49

28 questions · 30 min · instant report

Start

What interviewers check

Four things decide this round.

01

Queries

Joins, GROUP BY and HAVING, and anti-joins like “customers with no orders”.

02

Indexing

How compound indexes work and the write cost of every index.

03

Transactions

Isolation, race conditions and atomic updates.

04

Data modelling

When to embed versus reference in MongoDB.

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One question.
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This is the style of every question in the assessment: real situations, not trivia, with an explanation for each answer. The full test covers 28 questions across 7 skills.

Databases Not from the paid test

Which SQL clause keeps only customers with more than 5 orders after grouping?

WHERE filters rows before grouping; HAVING filters groups after aggregation: GROUP BY customer_id HAVING COUNT(*) > 5.

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Mistakes that cost offers

  • Adding an index for every column.
  • Read-modify-write updates that race under concurrency.
  • Copying a relational schema straight into MongoDB.

How to practise

Understand indexing (incl. compound indexes), joins, transactions/isolation and when to embed vs reference in MongoDB.

  • Model one domain in both PostgreSQL and MongoDB and compare queries.
  • Use EXPLAIN / explain() to prove an index is used.
  • Implement a safe money transfer with a transaction or atomic update.

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Nagendra Kumar

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Nagendra Kumar

IIT Delhi graduate · 10+ years

300+

engineers interviewed

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engineers hired

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FAQ

Questions, answered.

Often, yes. Expect SQL fundamentals plus at least one NoSQL store such as MongoDB or Redis, and when to use each.
Know how a B-tree index speeds reads, why compound index order matters, and that indexes slow writes.
The readiness assessment scores your database knowledge and gives a practice plan for any gaps.

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