SQL Window Functions Explained With Worked Examples
Learn ranking, running totals and comparisons across rows.
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Teach me SQL window functions using my own tables. Database system and version: [DATABASE]. Table names, columns and a few sample rows (invented or anonymised): [TABLES_AND_SAMPLES]. What I need to calculate: [GOALS], for example rank customers by spend within each region, running total of sales by date, difference from the previous month. Use only the tables and columns I listed; if you need more detail, ask. For each goal give the query in a code block followed by a small result table on my sample rows, and explain it clause by clause: the function, the partition, the ordering and the frame, in plain words. Show the arithmetic for one running total. Compare the window approach with GROUP BY or a self-join and say when each is better. Common mistakes: forgetting the partition, ties in rank functions (compare ROW_NUMBER, RANK and DENSE_RANK with two equal values), and frames that default to something surprising for running totals. Add three practice questions with answers. Test queries on a copy or read-only account, and mark dialect syntax [VERIFY].
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