Period-over-Period Comparisons

"How much did streams grow compared to last month?" "Which genres are trending year-over-year?" Period-over-period comparisons are one of the most common SQL interview patterns, especially at companies with SaaS, finance, or e-commerce products. The core challenge is straightforward: aggregate data into time periods, then compare each period to a previous one. The tricky part is handling missing periods, incomplete data, and choosing the right comparison technique.