SQL Challenges
SQL Quiz & Practice
The language every backend developer must know deeply.
Writing SQL that just works is one thing. Writing SQL that performs at scale is another. These challenges cover JOINs, CTEs, window functions, and NULL gotchas that appear in real data engineering interviews.
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5 Must-Know SQL Concepts
The topics that come up again and again in interviews — and in bugs.
JOIN Types
INNER, LEFT, RIGHT, FULL OUTER and when NULL rows appear.
Window Functions
ROW_NUMBER, RANK, LAG, LEAD, and PARTITION BY.
GROUP BY & HAVING
Aggregation logic and filtering aggregated results.
CTEs & Subqueries
WITH clauses, recursive CTEs, and correlated subqueries.
NULL Behaviour
Three-valued logic, IS NULL, COALESCE, and NULL in aggregates.
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