James BI 園地

I feel like I don't have the right to write about it after so many bits of information were moving in the web about this subject but I would like to add my point of view (or in fact - my point of code). First of all, I must mention some of those who written about this subject before me. Mosha Pasumansky wrote a long post about it in last may. This post contains some ideas of how to come along with this problem, but none of them is perfect. In fact (as always) - there is no perfect solution for this problem. Another important source of knowledge can be found here in the MSDN forums, where Chris Webb, Thomas Pagel and others discussed it. Now, I would like to add my solution. Take it or leave it - your choice.

First of all, create a column in the time dimension that will be the current day indicator. Thanks to the Data Source View or the UDM approach of SSAS 2005 you don't have to change the relational table itself. Just add a named calculation in the DSV with expression that will be 1 for the row of the current day and null or zero for the others. The expression syntax itself depends on the underlying DB so I won't write it here, but it's very simple. Add this column as an attribute (let's call it CurrentDayInd) in the dimension structure and select its AttributeVisible propertiey to false. That's because we don't need such Attribute Hierarchy in our time dimension. After that, create a new User Hierarchy (you can call it Current Day), where the first level in it will be CurrentDayInd and after it place the day (key) attribute.

Now, what we have here? We have a hierarchy (Current Day) with two members - 1 and zero. The 1 member has only one child which is the current day. Link that member to your regular hierarchy (it's called YSQMD at my place) and here you have it. For example, you can use it that way in the MDX script:

Create Set [Last 30 Days] as
LinkMember([Time].[Current Day].[CurrentDayInd].&[1].Children.Item(0),[Time].[YSQMD])
:
LinkMember([Time].[Current Day].[CurrentDayInd].&[1].Children.Item(0),[Time].[YSQMD]).Lag(30);

I think that this solution is good and elegant. First of all, you don't have to use external functions such as Now(). The second pro is that we use the native OLAP mechanism which saves time and makes the queries run faster. Believe me, you'll feel the difference with big cubes. The last thing is that this solution is easy to understand (at least I think so) and it is easy for maintenance. The big con is that you have to process this dimension (and related cubes) every day. I don't think that it's so bad because most of the organizations do make process every day.