Slowly altering dimension (SCD) is a knowledge warehousing idea coined by the superb Ralph Kimball. The SCD idea offers with transferring a selected set of knowledge from one state to a different. Think about a human sources (HR) system having an Worker desk. As the next picture reveals, Stephen Jiang is a Gross sales Supervisor having ten gross sales representatives in his group:

Right this moment, Stephen Jiang acquired his promotion to the Vice President of Gross sales position, so his group has grown in dimension from 10 to 17. Stephen is similar particular person, however his position is now modified, as proven within the following picture:

One other instance is when a buyer’s deal with modifications in a gross sales system. Once more, the shopper is similar, however their deal with is now totally different. From a knowledge warehousing standpoint, we’ve got totally different choices to cope with the info relying on the enterprise necessities, main us to various kinds of SDCs. It’s essential to notice that the info modifications within the transactional supply programs (in our examples, the HR system or a gross sales system). We transfer and rework the info from the transactional programs through ETL (Extract, Transform, and Load) processes and land it in a knowledge warehouse, the place the SCD idea kicks in. SCD is about how modifications within the supply programs replicate the info within the knowledge warehouse. These sorts of modifications within the supply system don’t occur fairly often therefore the time period slowly altering. Many SCD varieties have been developed over time, which is out of the scope of this submit, however on your reference, we cowl the primary three varieties as follows.

SCD sort zero (SCD 0)

With the sort of SCD, we ignore all modifications in a dimension. So, when an individual’s residential deal with modifications within the supply system (an HR system, in our instance), we don’t change the touchdown dimension in our knowledge warehouse. In different phrases, we ignore the modifications inside the knowledge supply. SCD 0 is additionally known as mounted dimensions.

SCD sort 1 (SCD 1)

With an SCD 1 sort, we overwrite the outdated knowledge with the brand new. A wonderful instance of an SCD 1 sort is when the enterprise doesn’t want the shopper’s outdated deal with and solely must maintain the shopper’s present deal with.

SCD sort 2 (SCD 2)

With the sort of SCD, we maintain the historical past of knowledge modifications within the knowledge warehouse when the enterprise must maintain the outdated and present knowledge. In an SCD 2 state of affairs, we have to keep the historic knowledge, so we insert a brand new row of knowledge into the info warehouse at any time when a transactional system modifications. A change within the transactional system is likely one of the following:

• Insertion: When a brand new row inserted into the desk
• Updating: When an current row of knowledge is up to date with new knowledge
• Deletion: When a row of knowledge is faraway from the desk

Let’s proceed with our earlier instance of a Human Useful resource system and the Worker desk. Inserting a brand new row of knowledge into the Worker dimension within the knowledge warehouse for each change inside the supply system causes knowledge duplications within the Worker dimensions within the knowledge warehouse. Due to this fact we can not use the EmployeeKey column as the first key of the dimension. Therefore, we have to introduce a brand new set of columns to ensure the individuality of each row of the info, as follows:

• A brand new key column that ensures rows’ uniqueness within the Worker dimension. This new key column is solely an index representing every row of knowledge saved in a knowledge warehouse dimension. The brand new secret’s a so-called surrogate key. Whereas the Surrogate Key ensures every row within the dimension is exclusive, we nonetheless want to keep up the supply system’s major key. By definition, the supply system’s major keys are actually referred to as enterprise keys or alternate keys within the knowledge warehousing world.
• A Begin Date and an Finish Date column characterize the timeframe throughout which a row of knowledge is in its present state.
• One other column reveals the standing of every row of knowledge.

SCD 2 is essentially the most frequent sort of SCD. After we create the required columns

Let’s revisit our state of affairs when Stephen Jiang was promoted from Gross sales Supervisor to Vice President of Gross sales. The next screenshot reveals the info within the Worker dimensions within the knowledge warehouse earlier than Stephen acquired the promotion:

The EmployeeKey column is the Surrogate Key of the dimension, and the EmployeeBusinessKey column is the Enterprise Key (the first key of the shopper within the supply system); the Begin Date column reveals the date Stephen Jiang began his job as North American Gross sales Supervisor, the Finish Date column has been left clean (null), and the Standing column reveals Present. Now, let’s take a look on the knowledge after Stephen will get the promotion, which is illustrated within the following screenshot:

Because the above picture reveals, Stephan Jiang began his new position as Vice President of Gross sales on 13/10/2012 and completed his job as North American Gross sales Supervisor on 12/10/2012. So, the info is reworked whereas transferring from the supply system into the info warehouse. As you see, dealing with SCDs is likely one of the most important duties within the ETL processes.

Let’s see what SCD 2 means with regards to knowledge modeling in Energy BI. The primary query is: Can we implement SCD 2 instantly in Energy BI Desktop with out having a knowledge warehouse? To reply this query, we should do not forget that we at all times put together the info earlier than loading it into the mannequin. Alternatively, we create a semantic layer when constructing a knowledge mannequin in Energy BI. In a earlier submit, I defined the totally different elements of a BI resolution, together with the ETL and the semantic layer. However I repeat it right here. In a Energy BI resolution, we maintain the ETL processes utilizing Energy Question, and the info mannequin is the semantic layer. The semantic layer, by definition, is a view of the supply knowledge (often a knowledge warehouse), optimised for reporting and analytical functions. The semantic layer is to not change the info warehouse and isn’t one other model of the info warehouse both. So the reply is that we can not implement the SCD 2 performance purely in Energy BI. We have to both have a knowledge warehouse preserving the historic knowledge, or the transactional system has a mechanism to help sustaining the historic knowledge, reminiscent of a temporal mechanism. A temporal mechanism is a characteristic that some relational database administration programs reminiscent of SQL Server supply to offer details about the info saved in a desk at any time as an alternative of preserving the present knowledge solely. To study extra about temporal tables in SQL Server, verify this out.

After we load the info into the info mannequin in Energy BI Desktop, we’ve got all present and historic knowledge within the dimension tables. Due to this fact, we’ve got to watch out when coping with SCDs. As an illustration, the next screenshot reveals reseller gross sales for workers:

At a primary look, the numbers appear to be appropriate. Properly, they could be proper; they could be unsuitable. It is determined by what the enterprise expects to see on a report. Take a look at Picture 4, which reveals Stephen’s modifications. Stephen had some gross sales values when he was a North American Gross sales Supervisor (EmployeeKey 272). However after his promotion (EmployeeKey 277), he’s not promoting anymore. We didn’t take into account SCD after we created the previous desk, which implies we take into account Stephen’s gross sales values (EmployeeKey 272). However is that this what the enterprise requires? Does the enterprise count on to see all workers’ gross sales with out contemplating their standing? For extra readability, let’s add the Standing column to the desk.

What if the enterprise must solely present gross sales values just for workers when their standing is Present? In that case, we must issue the SCD into the equation and filter out Stephen’s gross sales values. Relying on the enterprise necessities, we’d want so as to add the Standing column as a filter within the visualizations, whereas in different instances, we’d want to switch the measures by including the Begin Date, Finish Date, and Standing columns to filter the outcomes. The next screenshot reveals the outcomes after we use visible filters to take out Stephen’s gross sales:

Coping with SCDs isn’t at all times so simple as this. Generally, we have to make some modifications to our knowledge mannequin.

So, do all of the above imply we can not implement any kinds of SCDs in Energy BI? The reply, as at all times, is “it relies upon.” In some eventualities, we will implement an answer much like the SCD 1 performance, which I clarify in one other weblog submit. However we’re out of luck in implementing the SCD 2 performance purely in Energy BI.

Have you ever used SCDs in Energy BI, I’m curious to know in regards to the challenges you confronted. So please share you ideas within the feedback part beneath.