Aggregate it

A join gets you rows that belong together. Analysis is what happens next: deciding what one row of your *answer* should mean — one row per day, per driver, per city — and that decision is called the grain. Get it wrong and the query still runs, it just answers a question nobody asked. This chapter builds four increasingly analyst-shaped numbers off rides: a daily total, a ranking within a group, a share of a group's total, and a running total over time — the four things a window function is for. Every step is graded.