Creating a Semi-additive Measure over a Parent-child Hierarchy

Originally this blog post would have had the title "Disabling aggregations over a parent-child hierarchy", but I thought it could create confusion with the Aggregation Design concept in Analysis Services (SSAS), which is something completely different.

What I want to describe in this blog post is a method forcing a parent-child hierarchy in SSAS to show the parent's own data value, instead of the sum of the aggregated values of its children. Why would I want to do such a thing? A client of mine once asked me to create a cube on some financial data. However, the data was delivered by a 3rd party and could not be changed. This means that a parent-child hierarchy - for example the chart of accounts on a Profit&Loss report - should display the values from the database, not vpalues calculated or aggregated by SSAS. In most cases the values would have been the same, but sometimes it wouldn't, hence the need to create an alternative solution.