SQL Resources
Practical references built for practice, interviews, and everyday SQL work.
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SQL Master
A practical SQL reference for practice, interviews, and everyday analytics work.
Free learning guides
Focused explanations and study routines connected to hands-on SQL practice.
Aggregate rows confidently with COUNT, SUM, AVG, GROUP BY, and HAVING—plus the mistakes that commonly produce wrong totals.
Read guideLearn the practical difference between CTEs and subqueries, how to make multi-step SQL readable, and when neither improves performance.
Read guideUnderstand IS NULL, COALESCE, NULL-safe counting, and the three-valued logic behind many subtle SQL bugs.
Three SQL clauses you will use constantly, explained with practical examples, expected output, and common mistakes.
Read guideA beginner-friendly reference for choosing columns, filtering rows, combining conditions, and handling NULL correctly.
Read guideA realistic study routine that turns tutorials into query-writing skill and prepares you for analyst and engineering interviews.
Read guideA practical checklist of SQL topics and question patterns for data analyst, BI, and engineering interviews.
Read guideLearn ROW_NUMBER, RANK, LAG, LEAD, and running totals with a compact mental model and examples.
Read guideA practical guide to INNER, LEFT, RIGHT, FULL, CROSS, and self joins with common mistakes to avoid.
Read guideThe SQL skills data engineers use for reliable transformations, pipelines, data quality, and performance.
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