Being new to data warehousing, I never heard about the term "Slowly Changing Dimensions" before. It sounded like academic jargon. It isn't. It's one of those concepts that, once it clicks, explains half the weird design decisions you'll see in any reporting database. This post is the explainer I wish existed before I had to learn it the hard way: by breaking a report Facts and dimensions, the short version Data warehouses generally split data into two kinds of tables. Fact tables hold the things that happened: an order, a sale, a support ticket, a click. They're typically just numbers and foreign keys — quantity, amount, a reference to which customer, which product, which date. Dimension tables hold the descriptive context around those facts: who the customer is, what the product is called, which region a salesperson covers. A fact on its own is nearly meaningless. "47 units, customer 1182, product 309" tells you nothing until you jo...