You wrote the query in an afternoon. It ran against your development database in four milliseconds, the test suite went green, and you shipped it. Six weeks later, someone in support forwards a screen...
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...