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SQL Articles & Tutorials
Expert-written tutorials, guides, and deep dives into SQL — from basics to advanced optimization. Direct answers, examples, and FAQs.
// LATEST ARTICLES37 ARTICLES
Market Basket Analysis SQL Interview Explained
This is the retail and e-commerce interview classic: find product pairs frequently bought together. One line of self-join logic makes or breaks the answer.
Year-over-Year Growth SQL Interview Walkthrough
YoY growth questions test whether a candidate reaches for LAG or a clunky self-join, and whether they remember to guard against divide-by-zero. A full walkthrough with both engines.
User Retention SQL Interview Question: Month-1 Retention
Retention questions are a growth team staple in SQL interviews. This walks through the exact query, the schema, and the integer division bug that quietly wrecks an otherwise correct solution.
SQL Window Functions: Rolling Averages Interview
Window functions are the single most common topic in SQL interviews that separates candidates who understand real analytics from those who only know basic SELECT statements. Here is the exact rolling average question interviewers ask, solved in SQLite and PostgreSQL.
The Ultimate SQL Cheat Sheet
A single-page SQL reference covering JOINs, window functions, CTEs, NULL handling, CASE WHEN, string/date functions, constraints, common query patterns, and more.
SQL Injection: How It Works and How to Prevent It
Learn how SQL injection attacks actually work — authentication bypass, UNION-based data exfiltration — and how parameterized queries prevent them structurally.
Idempotent SQL: Writing Safe, Re-runnable Scripts
Learn to write idempotent SQL for migrations, seed scripts, and ETL jobs — safe upserts, guarded schema changes, and the relative-update trap that compounds on re-run.
RFM Analysis and Customer Segmentation in SQL
Learn to build Recency, Frequency, Monetary (RFM) customer segmentation in SQL using NTILE, including the recency-inversion gotcha and segment-level reporting.
A/B Test Analysis with SQL
Learn to analyze A/B tests in SQL — conversion rate by variant, sample ratio mismatch checks, a two-proportion z-test, and common statistical pitfalls like peeking.
CTE vs Subquery vs Temp Table vs View: When to Use Each
A practical decision framework for choosing between a CTE, subquery, temp table, view, or materialized view in SQL, with a comparison table and real tradeoffs.