Guides

SQL Guides

Concept guides that explain the SQL patterns behind interview questions — then let you practice them with real data.

SQL Window Functions: The Complete Guide
Learn ROW_NUMBER, RANK, DENSE_RANK, LAG, LEAD, and aggregate windows with worked SQL interview examples, frame definitions, and the mistakes that fail interviews.
SQL JOINs Explained: Inner, Left, Self, and Anti-Joins
Understand INNER JOIN, LEFT JOIN, self joins and anti-joins through SQL interview questions, with the double-counting and NULL pitfalls that trip up candidates.
SQL Subqueries and CTEs: A Practical Guide
Master scalar subqueries, derived tables, NOT IN vs NOT EXISTS, and WITH clauses through real SQL interview patterns.
SQL GROUP BY and HAVING: Patterns for Interview Questions
Learn the GROUP BY / HAVING patterns behind duplicate detection, threshold filtering, and per-group aggregation with real examples.