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You can write SELECT, JOIN and GROUP BY, yet the real requests keep arriving: the top orders in every region, a running total that restarts each month, a moving average, the longest streak of daily logins, everyone under a given manager at any depth. This book teaches the patterns behind those reports. You learn window functions with ranking, frames, lag and lead, then common table expressions and recursive queries for hierarchies, calendars and route searches. Later chapters cover gaps and islands, safe deduplication, top-N per group, pivots with conditional aggregation and set operations. Every query was run on a small fictional shop database and the real result rows are shown, with plain notes on where database engines differ, so you can check each answer by eye.
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