Your manager sends a one-line request on a Thursday afternoon: "Can you pull the top three orders for every customer, plus how each one compares to the previous order?" You open your SQL editor, stare...
It's a Tuesday afternoon, and the dashboard that loaded instantly during your first month of launch now takes eleven seconds. Nothing in the code changed. The table just grew from 5,000 rows to 4 mill...
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...