1.The Notebook Problem

Every evening, Mike closes GreenMart the same way. Mike pulls out a notebook and writes down the day — today's sales, who paid, who didn't, what Sarah restocked. This has worked for years. But it's about to fall apart.

A regular customer walks in and asks a simple question.

Mike flips back through old pages — March, February, a coffee stain over January. Three different numbers show up for the same thing, on three different days. None of them match. Nobody wrote down which one was right.

This isn't really a notebook problem. It's a data problem. And it helps to be clear about what "data" actually means — everything from here on depends on getting that right.

Everything Mike wrote down is data — a fact, recorded somewhere. "Priya, 340" is data. "Priya paid 200" is data too. But a number like "340" means nothing by itself. It only becomes useful once something tells you:

Mike's notebook has plenty of data in it. What it's missing is a reliable way to say which piece of data is true right now.

That's the real problem a database solves. Not "where do I write things down" — but "how do I keep facts organized, so months later they still mean exactly what they meant the day I wrote them."