SQL Interview Questions & Answers

Master your SQL interviews with 50 questions across six topics, each with a detailed, worked answer.

50 Questions

6 Topics

4 Difficulty Levels

SQL Fundamentals Interview Questions Core SQL concepts every interview starts with — NULLs, keys, constraints, data types and aliases. 8 questions Open SQL JOIN Interview Questions INNER, OUTER, CROSS and FULL joins — how they differ and when to reach for each. 4 questions Open SQL Query Interview Questions Query operations interviewers love — WHERE vs HAVING, UNION, subqueries, CASE, IN vs EXISTS and more. 9 questions Open SQL Functions Interview Questions Aggregate, window, string, date and analytic functions, with worked examples. 8 questions Open SQL Performance and Optimization Interview Questions Execution plans, indexing, normalization, locking, partitioning and caching. 10 questions Open Advanced SQL Interview Questions Transactions and ACID, CTEs, stored procedures, triggers, views, sharding and cursors. 11 questions Open

Every question

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Fundamentals 8

What is SQL and what is it used for? Beginner
What is a Primary Key and Foreign Key? Intermediate
What is database normalization and what are its normal forms? Expert
What is NULL in SQL and how is it different from empty string or zero? Beginner
What is the difference between clustered and non-clustered indexes? Beginner
What are SQL constraints and what types exist? Beginner
What are SQL aliases and why are they used? Beginner
What are SQL data types and their common uses? Beginner

Joins 4

Explain different types of JOINs in SQL Intermediate
What is a CROSS JOIN and when would you use it? Intermediate
What is a FULL OUTER JOIN and how does it differ from other joins? Advanced
What is the difference between INNER JOIN and OUTER JOIN? Intermediate

Queries 9

What is the difference between WHERE and HAVING clauses? Beginner
Explain the difference between UNION and UNION ALL Advanced
What is the difference between DELETE, TRUNCATE, and DROP? Intermediate
What are correlated subqueries and how do they work? Advanced
What is the difference between IN and EXISTS operators? Intermediate
What is the CASE statement and how is it used? Intermediate
What is the difference between UNION and JOIN? Intermediate
What are recursive queries and how are they written? Intermediate
What is the difference between ANY, ALL, and SOME operators? Intermediate

Functions 8

What are Window Functions and how do they differ from GROUP BY? Advanced
What are aggregate functions in SQL? List commonly used ones. Intermediate
What is the difference between RANK(), DENSE_RANK(), and ROW_NUMBER()? Intermediate
What are string functions in SQL? Provide examples. Intermediate
What are date and time functions in SQL? Provide examples. Intermediate
What are PIVOT and UNPIVOT operations in SQL? Intermediate
What are SQL analytic functions vs aggregate functions? Intermediate
What are SQL mathematical and statistical functions? Intermediate

Performance 10

What is query execution plan and how do you read it? Intermediate
What is database normalization and denormalization? When to use each? Advanced
What is database locking and what are different lock types? Advanced
What is query caching and how does it work? Advanced
What is database partitioning and what are its benefits? Advanced
What are covering indexes and how do they improve performance? Advanced
What is database connection pooling and its benefits? Advanced
What is database connection multiplexing and its benefits? Advanced
What is query plan caching and parameter sniffing problem? Advanced
What is database connection failover and high availability? Advanced

Advanced 11