You've written a query that pulls every customer in your database. It runs fine. Then someone asks a simple-sounding question: which customers placed an order last month, and what did they spend? Sudd...
You already know how to write a `GROUP BY`. You can total up sales by month, count users by country, average an order value without breaking a sweat. Then someone asks a question that sounds just as s...
You have a table of sales, and your boss asks a deceptively simple question: "For each salesperson, show me their monthly total and how that compares to the month before." You know how to get the mont...
