Tod means Fox

This article is part of a series discussing the Kimball Group’s “34 Subsystems of ETL“. The Subsystems are a group of “Best Practices” for delivering a BI/DW solution. In my articles, I discuss how each Subsystem can be implemented in SSIS or hand coded in Visual FoxPro.

This subsystem exists to ensure that hierarchies are appropriately translated and represented in the dimensional model. There are two types of hierarchies that you’ll need to contend with: fixed and ragged.

Managing hierarchies can be a complex process especially when you have hierarchies that are extremely ragged (for example, manufacturers’ parts or an organizational chart). You’ll also run into complexities when a dimension entity (like a single customer or a single product) exists in simultaneous hierarchies. I’ll talk more about these later in this post. In contrast, the fixed variety is easier to work with.

Hierarchies as Attributes of a Dimension

Generally speaking, you can think of hierarchies as many-to-one relationships. In the dimensional modeling world, these relationships are represented in a single table. This would include stores to regions to state, children to parents to grandparents, and greens to vegetables to produce. ETL Subsystem 11 seeks to maintain the integrity of these relationships.

To simplify even further, instead of thinking in “tree” structures, think in “lines” from the child up the hierarchy to the parent. This will help you build the dimensions to accommodate the relationships. In the simplest form: A single store is in a single region in a single state. This is actually a very interesting topic from a modeling perspective — one in which I’ll get to in more detail in a future posting.

Dimensions are denormalized structures, which means that you will have many repeated elements. This is normal and desired. For example, a store region will be repeated for each store and state will be repeated for each region all in the same dimension (look left). This is normal and desirable in a dimension. The trick to getting this to work correctly is that the hierarchy must be represented as a single value with the dimension row’s primary (surrogate) key.

Data modelers who are used to 3rd Normal Form might look at the above and cringe. But remember: normalized models are for preventing data anomalies in a transaction environment. In a data warehousing environment, the rules are different. First, there are no opportunities for data anomalies due to data integration controls. Secondly, normalizing the data warehouse makes absolutely no sense from a usability perspective: it only complicates and slows down reports and queries.

The Ragged Kind

So fixed hierarchies are easiest to work with: got it. It is not so easy to work with variable and ragged hierarchies because of their variable depth. The classic example is an organization chart, where any employee can be at the top or at the bottom of the hierarchy. Knowing how deep the organization runs from any point is a challenge that usually requires self joins and bridge tables to represent the relationships.

I have always solved these types of hierarchy “problems” using “helper tables” in a dimensional model. Ralph Kimball wrote a great article a decade ago on this subject. Check it out for more details.

Helper tables look like bridge tables that sit between the fact and the dimension. They facilitate the representation complex hierarchal information. This design complicates user queries though, so be sure that helper tables are absolutely needed. It might be, for example, you only really need the manager’s name and not the entire chain of command with each employee. You don’t need a helper table for that (see the following code)!

SELECT
   c.FirstName AS empFirstName,
   c.LastName AS empLastName,
   e.title AS empTitle,
   COALESCE(m.mgrFirstName,'N/A'),
   COALESCE(m.mgrLastName,'N/A'),
   COALESCE(m.mgrTitle,'I am the boss')
FROM
    HumanResources.Employee e 
JOIN Person.contact c ON c.ContactId = e.EmployeeID 
LEFT JOIN (
   SELECT
      e2.EmployeeID,
      c2.FirstName AS mgrFirstName,
      c2.LastName AS mgrLastName,
      e2.title AS mgrTitle 
   FROM
      HumanResources.Employee e2 
   JOIN Person.contact c2 ON c2.ContactId = e2.EmployeeID  ) 
  AS m ON m.EmployeeID = e.ManagerID

I wrote a while ago on ragged hierarchies from a programming perspective. Take a look at that post for more details.

Date Hierarchies

For another example, let’s look at a common hierarchy we can all relate to: year, quarter, month, and week. You can see how this hierarchy is modeled by looking at any date dimension. As you’ll see, each day contains information about how it is grouped on the calendar. The information is repeated for each day until one of the groups change. When designing reports that allow your users to drill down into the data, it is a common approach to start at the highest group (sales by year) and then look a bit deeper as necessary (sales by quarter or sales by month, for example).

Also, in the date dimension, weeks don’t line up too well within a year, quarter, or month. This is a classic example of how one group can fit entirely or partly into another. You see this often when a sales region crosses multiple states, or when an employee serves multiple roles within the company. These are all situations that must be accounted for by your hierarchy manager. For the date dimension, one technique I’ve adopted is to include the week of the year number, week of the quarter, and week of the month in the date dimension to give users the ability to drill into weekly data much easier (for example, you might want to measure holiday sales in the US from the 3rd week of November to the 4th week of December).

Snowflakes and Hierarchies

Snowflakes are usually a sign that a hierarchy has been normalized. This is bad. Don’t fall into the trap! In order to keep the dimensional model as simple as possible, you should avoid snowflaking; however, snowflake designs are perfectly legal under certain circumstances. Carefully examine the reasons though. If you are snowflaking to accommodate a hierarchy, hold your horses. Hierarchies are a natural part of a dimension. In fact, most things in life are categorized and need to be grouped in some way. Remember that one of the primary purposes of delivering a denormalized dimension is to remove almost all complexity from the user’s perspective. This usually means hierarchies.

SQL Server 2005 Integration Services (SSIS)

Denormalizing hierarchies using SSIS is not difficult. The hardest part is usually in writing the SQL that correctly fetches the right data. In a Data Flow, you can use a series of Lookup and Merge components. In some cases, especially for the more complex ragged hierarchies, your best bet is to use SQL statement in an Execute SQL Task.

Although I won’t get into details in this post, CTEs (Common Table Expressions) are excellent for working with recursion and hierarchies.

The real joy of hierarchy management using the SQL Server Business Intelligence suite comes at the end when you want to start using your hierarchies to allow users to drill down into the data. Even if you have complex ragged hierarchies structured with bridge tables in your model, Analysis (SSAS) and Reporting (SSRS) Services can really make sense of it all. But I’m digressing; this series of posts is about ETL and not presentation tools!

Hand Coding with Visual FoxPro (VFP9)

It’s easy to write recursive functions in Visual FoxPro. Recursion is one of the “secretes” to flattening out hierarchal structures. For more details on writing recursive functions in FoxPro, check out my post “Ragged Hierarchy Alert” or even better, visit the FoxWiki page on the subject.

From Here

In the end, as I continue to compare SSIS with VFP on data integration, I find that hierarchy management is about equal between the two. FoxPro seems to perform much better but I have no benchmarking (yet) to prove it. One advantage with SSIS is being able to utilize CTEs.

This was a long one, but a heavy subject. In my next post, I’ll discuss the Special Dimensions Manager, ETL Subsystem 12. Not as heavy and hopefully not as long!