DAX (Data Analysis Expressions) is the formula language of Power BI (and Excel Power Pivot) with which you define calculations over the data model: revenue compared to last year, moving averages, margins per segment. Where Power Query prepares data, DAX calculates with it.
DAX in practice
DAX looks like Excel formulas but works in a fundamentally different way: a formula does not calculate over one cell but over the whole model, within the 'filter context' of the chart it sits in. The same measure automatically shows the right figure per month, per customer or per region — that is its power, and also its learning curve.
For most SMB dashboards a limited set of well-written DAX measures is enough; complexity lies more in correct definitions (what exactly is 'revenue'?) than in exotic functions. The division of roles between the model, Power Query and DAX is reflected in What does a Power BI dashboard cost?
Related terms
- Power Query — Power Query is the tool in Excel and Power BI with which you retrieve, clean and reshape data from sources — without programming.
- Data model — A data model is the structure in which data and its mutual relationships are captured: which tables there are (customers, orders, products), which fields they contain and how they are linked together.
- BI — BI (business intelligence) is the collecting, combining and visualising of business data to base decisions on — usually in the form of dashboards and reports.
- KPI — A KPI (key performance indicator) is a measurable figure that shows how an organisation or process is performing on something that really matters — revenue per customer, delivery time, time-to-fill, error rate.