SQL Window Functions Explained: PARTITION BY, RANK, LAG and More

SQL window functions allow calculations across a set of rows while preserving each individual row in the result, unlike GROUP BY which collapses records into groups. Functions such as ROW_NUMBER() and RANK() enable row ranking within a dataset, with RANK() assigning equal ranks to tied values while ROW_NUMBER() always produces unique sequential numbers. PARTITION BY divides data into subgroups before a calculation is applied, making it possible to compute metrics like department-level averages alongside employee-level detail. LAG() and LEAD() retrieve values from preceding or following rows respectively, enabling row-to-row comparisons such as tracking changes in daily sales figures. Together, these functions extend SQL's analytical capability beyond simple aggregation, making them valuable for tasks like running totals, trend analysis, and ranked reporting.