Your app was fast for a year. Nothing changed — same code, same server, same query you wrote in an afternoon and never thought about again. Then one Tuesday the orders page takes eleven seconds to loa...
You wrote a query to list your customers and their orders. It ran fine — but half your customers vanished from the results. The ones who never placed an order simply weren't there. Nothing errored, no...
You wrote a query. It ran. And then it returned 40,000 rows when your `customers` table only has 8,000. Or worse: it returned nothing at all, even though you can see the matching records right there i...
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...