Window Functions: SQL's Most Powerful Feature Nobody Uses

You are going to hit a problem. Not now. A few weeks into data analysis work, or maybe during a job interview, someone will ask you something like: "For each sale, can you show the sale amount alongside the running total for that salesperson?" Or: "Rank employees by salary within each department without removing any rows." Or: "For each order, show the previous order's amount for comparison." You will try GROUP BY. It collapses rows. Wrong. You will try joins. Awkward, multiple CTEs. Messy. You