What is a Type 2 change in a dimension table?

What’s the Deal with Type 2 Changes in Dimension Tables? (And Why Should You Care?)

Okay, so you’re building a data warehouse. Awesome! You’ve got your fact tables loaded with all the juicy numbers, but what about the context? That’s where dimension tables come in – think of them as the who, what, where, and when that give your data meaning. But here’s the thing: the who, what, where, and when change. People move, products get updated, and well, you get the picture. How you handle those changes is crucial, and that’s where the concept of “Slowly Changing Dimensions” (SCDs) enters the chat.

Now, there are different flavors of SCDs, but today, we’re diving deep into Type 2. In my opinion, it’s the most useful and widely used of the bunch. Forget updating the existing information; with Type 2, every time something changes, you create a new record. Think of it like keeping a historical snapshot of your data over time.

Why bother with all this extra work? Simple: history matters!

Let’s say you want to track customer behavior. Knowing that John Doe lives at 123 Main Street right now is good, but what if he lived at 456 Oak Avenue last year? Maybe customers who moved from Oak Avenue are more likely to churn. Without Type 2 SCDs, you’d never know!

Here’s why Type 2 SCDs are your best friend:

• Spotting Trends: Ever tried to figure out why sales spiked in a certain region? Maybe it was because a popular store opened there. Type 2 SCDs let you see how things like store locations change over time, helping you connect the dots.
• Accurate Historical Reporting: Imagine your boss asks for a report on sales from last quarter. But product prices changed mid-quarter! With Type 2 SCDs, you can be sure you’re using the right prices for that specific time period. No more fudging the numbers!
• Keeping the Regulators Happy: Sometimes, you’re legally required to keep a record of changes. Type 2 SCDs make compliance a whole lot easier.

So, how does this magic work? It’s actually pretty straightforward:

A few things to keep in mind when using Type 2 SCDs:

• Surrogate Keys are a Must: Seriously, don’t even try this without them.
• Storage Space: Keeping all that history takes up space. Be prepared for your dimension tables to grow, especially if you have dimensions that change frequently.
• Query Optimization: Getting the right data out of a Type 2 SCD requires some careful query writing. You need to make sure you’re grabbing the correct version of the record for the time period you’re interested in. Indexing and partitioning can be lifesavers here.
• ETL Complexity: Getting all this set up in your ETL process can be a bit tricky. You need to be able to detect changes and insert/update records correctly.

Let’s go back to our customer example. Imagine you have a customer table with these details:

• CustomerID (Your unique, surrogate key)
• CustomerKey (The customer ID from your source system)
• Name
• Address
• StartDate
• EndDate
• IsActive

Initially, your table might look like this:

CustomerIDCustomerKeyNameAddressStartDateEndDateIsActive1123John Doe123 Main St2023-01-019999-12-311