SQL Window Functions
Deep Dive.

Build the mental model: every window function is a regular function with three dials — PARTITION (where to apply), ORDER BY (in what sequence), and a frame (how big a slice).

Master ROW_NUMBER, RANK, DENSE_RANK, and NTILE. Pick the right one for ties, leaderboards, tiers, and bucketing — and nail the top-N-per-group interview pattern.

Take SUM, AVG, COUNT, MIN, MAX out of GROUP BY and into OVER. Running totals, percent-of-total, and multiple windows in one query.

Compare a row to its previous or next sibling. The foundation of period-over-period growth, churn detection, and gap/sequence analysis.

Frames decide which rows the window sees. ROWS counts physical positions, RANGE counts logical values — and the default frame quietly bites everyone once.

The SQL standard is one thing, vendor implementations are another. Learn what SQLite skips, what PostgreSQL extends with (FILTER, IGNORE NULLS), and how to write portable analytics.

Beyond ranking and aggregation: FIRST_VALUE, LAST_VALUE (and its trap), NTH_VALUE, PERCENT_RANK, CUME_DIST, and conditional aggregation with FILTER.

Windows beat self-joins for N-th-row problems, but misuse blows up intermediate sets. Read EXPLAIN, spot the anti-patterns, and refactor slow analytics queries.