What is Data Analysis Expressions (DAX)?
Data Analysis Expressions aka DAX allows you to calculate, create and return information on your data. Using DAX enables you to add more context to your original data to perform better analysis and get more meaning from your data.
DAX formulas are similar to writing functions in Excel and can be used in Analysis Services, Power BI, or Power Pivot in Excel.
Data Analysis Expressions formulas can be used to create measures, calculated columns, calculated tables or filter rows.
For examples of how to build Analysis Services Tabular models and leverage DAX see my book.
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