The notion of time pervades every corner of the data warehouse. Most of the fundamental measurements we store in our fact tables are time series, which we carefully annotate with time stamps and foreign keys connecting to calendar date dimensions. But the effects of time are not isolated just to these activity-based time stamps. All […]

The owner of the data warehouse must decide how to respond to the changes in the descriptions of dimensional entities like Employee, Customer, Product, Supplier, Location and others. In 30 years of studying this issue, I have found that only three different kinds of responses are needed. I call these slowly changing dimension (SCD) Types […]

How do you deal with changing dimensions? Hybrid approaches fill gaps left by the three fundamental techniques. Unlike most OLTP systems, a major objective of a data warehouse is to track history. So, accounting for change is one of the analyst’s most important responsibilities. A sales force region reassignment is a good example of a […]

Ralph introduced the concept of slowly changing dimension (SCD) attributes in 1996. Dimensional modelers, in conjunction with the business’s data governance representatives, must specify the data warehouse’s response to operational attribute value changes. Most Kimball readers are familiar with the core SCD approaches: type 1 (overwrite), type 2 (add a row), and type 3 (add […]

Most ETL tools provide some functionality for handling slowly changing dimensions. Every so often, when the tool isn’t performing as needed, the ETL developer will use the database to identify new and changed rows, and apply the appropriate inserts and updates. I’ve shown examples of this code in the Data Warehouse Lifecycle in Depth class using standard INSERT […]

Meaningless integer keys, otherwise known as surrogate keys, are commonly used as primary keys for dimension tables in data warehouse designs. Our students frequently ask us – what about fact tables? Should a unique surrogate key be assigned for every row in a fact table? Although for the logical design of a fact table, the answer is no, […]

Drawing the Line Between Dimensional Modeling and ER Modeling Techniques Dimensional modeling (DM) is the name of a logical design technique often used for data warehouses. It is different from, and contrasts with, entity-relation modeling (ER). This article points out the many differences between the two techniques and draws a line in the sand. DM […]

One of the tasks of the ETL system’s customer dimension manager is to “assign a unique durable key to each customer.” By durable key, we mean a single key value that uniquely and reliably identifies a given customer over time. In most cases, this unique durable key is the natural business key from the operational […]

The importance of the time dimension in data marts and data warehouses. The time dimension is a unique and powerful dimension in every data mart and enterprise data warehouse. Although one of the tenets of dimensional modeling is that all dimensions are created equal, the truth is that the time dimension is very special and […]

Dimensional modeling is a design discipline that straddles the formal relational model and the engineering realities of text and number data. Compared to entity/relation modeling, it’s less rigorous (allowing the designer more discretion in organizing the tables) but more practical because it accommodates database complexity and improves performance. Contrasted with other modeling disciplines, dimensional modeling […]

Data warehousing has never been more valuable and interesting than it is now. Making decisions based on data is so fundamental and obvious that the current generation of business users and data warehouse designers/implementers can’t imagine a world without access to data. I’ll resist the urge to tell stories about what it was like before […]

Do you know the difference between dimensional modeling truth and fiction? According to Merriam-Webster, fables are fictitious statements. Unfortunately, fables about dimensional modeling circulate throughout our industry. These false claims and assertions are a distraction, especially if you’re trying to align a team. In this column, we’ll describe the root misunderstandings that perpetuate these myths so […]