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Database Engineer Interview Guide

SQL and Database Interview Questions

Screen candidates on schema design, query performance, indexing, transactions, and operational database judgment.

Strong SQL candidates explain trade-offs between normalization, indexing, locking, and operational safety. These prompts test practical judgment, not memorized syntax.

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The signal behind strong database engineer answers.
Section 1

Schema Design

Check normalization, access patterns, and migration awareness.

1. Design the schema for a multi-tenant SaaS with hard isolation between tenants.

Interview prompt

Strong signal

Discusses tenant column vs schema-per-tenant, indexing strategy, and migration safety.

Follow-up probes

  • •What query patterns become risky?
  • •How do you handle noisy neighbors?

Red flags

  • •Picks one model without trade-off discussion.
  • •Ignores backups or migrations.
Section 2

Query Performance

Evaluate index choice, query plans, and measurement.

1. A reporting query takes 30 seconds. Walk me through how you would investigate and improve it.

Interview prompt

Strong signal

Reads EXPLAIN, identifies scans vs seeks, considers indexes, materialized views, and partitioning.

Follow-up probes

  • •What if the table is 1B rows?
  • •How do you avoid breaking writes?

Red flags

  • •Adds indexes blindly.
  • •Ignores write impact.
Section 3

Transactions

Test isolation levels, locking, and concurrency reasoning.

1. Two services update the same row concurrently and one update is silently lost. What is happening?

Interview prompt

Strong signal

Mentions read-modify-write hazards, isolation levels, optimistic locking, and version columns.

Follow-up probes

  • •When is serializable too expensive?
  • •How do you test for this?

Red flags

  • •Cannot describe isolation levels.
  • •Suggests global locks as default.

Hire better with structured interviews

Put a candidate through a structured AI or human-led round and get a reportcard with the transcript and the code attached.