Much has been said about DAX, and also many books have been written, how DAX can be used to answer questions. Nevertheless DAX is an essential part of one of my current favorite analytical tools, namely MS SQL Server Analysis Services BISM Tabular in it's flavor of Power BI (flavor does not mean it's a special dialect like T-SQL or PSQL are dialects of SQL), from all the tools that rely on the tabular data model, Power BI is the one that gets updates meaning new features and capabilities more frequently than the other tools.
For this reason I want to add some writing to this absolutely incredible query language, and this is just to spread the word, to level up the probability, that someone who has issues with DAX stumbles across this site, and it helps to resolve this issue.
Basically I think that it's necessary to understand the mechanics of DAX to tackle more advanced challenges, for this reason I divide my writings into two categories "DAX Foundation", and "More Advanced DAX". More Advanced DAX statement do not necessarily mean that more lines of code are written, sometimes it exactly has this meaning, but it means that complex questions can only be answered through a deeper understanding of the foundations.