🗄️ Database Interview Questions

Master database interviews with questions on SQL, NoSQL, optimization, indexing, and database design

Explain database indexing and when to use different index types

Hard

Indexes improve query performance by creating data structures that allow faster lookups. However, they slow down writes and consume additional storage.

Common Index Types:

  • B-Tree: Default index, good for equality and range queries
  • Hash: Fast for equality comparisons, not for ranges
  • GiST/GIN: Full-text search and complex data types
  • Partial: Index on subset of rows
  • Composite: Index on multiple columns
SQL

Explain ACID properties and implement a database transaction

Medium

ACID guarantees reliability of database transactions:

  • Atomicity: All operations succeed or all fail (no partial updates)
  • Consistency: Data remains valid according to rules/constraints
  • Isolation: Concurrent transactions don't interfere with each other
  • Durability: Committed data persists even after system failure
SQL

Compare SQL vs NoSQL databases and when to use each

Medium

SQL (Relational) and NoSQL (Non-relational) databases have different strengths:

SQL (PostgreSQL, MySQL)

  • Structured data, fixed schema
  • ACID transactions
  • Complex queries, JOINs
  • Vertical scaling
  • Strong consistency

NoSQL (MongoDB, Redis)

  • Flexible/dynamic schema
  • BASE (Basically Available, Soft state, Eventual consistency)
  • Denormalized data
  • Horizontal scaling
  • High performance for specific use cases
SQL
JavaScript
JavaScript

Explain database normalization and denormalization with examples

Medium

Normalization: Organizing data to reduce redundancy and improve integrity.Denormalization: Intentionally introducing redundancy for performance.

Normal Forms:

  • 1NF: Atomic values, no repeating groups
  • 2NF: 1NF + no partial dependencies
  • 3NF: 2NF + no transitive dependencies
  • BCNF: 3NF + stricter rules
SQL

Explain database query optimization techniques

Hard

Query optimization improves performance by reducing execution time and resource usage.

SQL

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