SQL Guides & Interview Prep

Subqueries appear in almost every SQL interview. This guide explains non-correlated vs correlated subqueries, scalar subqueries, EXISTS vs IN, and exactly when to use each — with real interview examples throughout.

String manipulation in SQL is tested more than most candidates expect. This cheat sheet covers CONCAT, TRIM, UPPER, LOWER, SUBSTRING, REPLACE, LIKE, and the string patterns that keep appearing in data analyst interviews.

These 10 SQL mistakes silently kill candidates in data interviews. Most are not about not knowing SQL — they are about how you think, communicate, and handle edge cases under pressure. Here is how to fix every one of them.

Date functions trip up more SQL candidates than almost any other topic. This cheat sheet covers DATE_TRUNC, DATEDIFF, EXTRACT, date arithmetic, and the time-series patterns that appear in every data analyst interview.

SQL CASE WHEN is the most versatile tool in your SQL toolkit. This guide covers simple vs searched CASE, conditional aggregation, CASE in ORDER BY, and the interview patterns that keep coming up at top companies.

Learn SQL first. SQL is required in nearly every analyst job, faster to learn, and tested in virtually every interview. Here is exactly when to learn each skill and the recommended learning path to land your first data analyst role.

A step-by-step framework for every SQL interview question — how to read a schema fast, plan your query, write clean SQL using CTEs, and verify your result. Includes the 6 most common SQL interview patterns and the mistakes that eliminate most candidates.

Learning SQL to a job-ready level takes 4 to 12 weeks depending on your starting experience. Complete beginners need 10–12 weeks. People with some data background can be interview-ready in 4–6 weeks with focused daily practice.

The complete SQL topic checklist for data analyst interviews — covering joins, aggregations, window functions, CTEs, subqueries, NULL handling, and date functions. Know exactly what to study and what to skip.

Practising SQL for a data analyst interview means solving real-world problems that mirror what companies actually test — not just memorising syntax. This guide gives you a step-by-step practice routine, the core topics to focus on, and how long it takes to get interview-ready.

Confused about when to use INNER JOIN vs LEFT JOIN? This practical guide breaks down every type of SQL JOIN with real examples, common mistakes to avoid, and interview tips so you always pick the right join the first time.

Never written SQL before? This beginner guide takes you from zero to writing real SELECT, WHERE, JOIN, and GROUP BY queries in 30 minutes – with practical examples on tables you might actually work with on the job.

NULLs are responsible for more silent SQL bugs than almost anything else. This guide explains what NULL really means, why = NULL never works, how NULLs affect aggregates and JOINs, and the functions you need to handle them correctly every time.

GROUP BY trips up more SQL candidates than almost any other concept. This guide explains exactly how aggregation works, why HAVING exists, common mistakes that cause wrong answers, and the patterns that keep appearing in data interviews.

Should you use a CTE or a subquery? This guide breaks down the real differences between WITH clauses and nested SELECTs, when each shines, and how to answer this question confidently in any SQL interview.

Preparing for a SQL interview? This guide walks you through 25 realistic query patterns – from filtering and joins to window functions and time-series analytics – each mapped to a hands-on QueryLab problem so you can practice with real data, not just theory.

Preparing for a SQL interview? This practical cheat sheet shows you the 21 query patterns that keep appearing in data analyst and data engineer interviews – with examples, tips, and mistakes to avoid.

Confused by SQL window functions? This practical cheat sheet shows you how ROW_NUMBER, RANK, DENSE_RANK, LAG, LEAD, and running totals really work – with examples you can reuse in interviews and on the job.

Slow SQL queries killing your app? This practical guide explains clustered vs non‑clustered indexes, composite keys, covering indexes, and common anti‑patterns – with examples you can apply directly to production databases and interviews.