Data Analysis Expressions (DAX) is a formula expression language used in Analysis Services, Power BI, and Power Pivot in Excel. DAX formulas include functions, operators, and values to perform advanced calculations and queries on data in related tables and columns in tabular data models.
Calculations
DAX formulas are used in measures, calculated columns, calculated tables, and row-level security.
Measures
Measures are dynamic calculation formulas where the results change depending on context.
Measures are used in reporting that support combining and filtering model data by using multiple attributes such as a Power BI report or Excel PivotTable or PivotChart.
Measures are created by using the DAX formula bar in the model designer.
A formula in a measure can use standard aggregation functions automatically created by using the Autosum feature, such as COUNT or SUM, or you can define your own formula by using the DAX formula bar.
Named measures can be passed as an argument to other measures.When you define a formula for a measure in the formula bar, a Tooltip feature shows a preview of what the results would be for the total in the current context, but otherwise the results are not immediately output anywhere.
The reason you cannot see the (filtered) results of the calculation immediately is because the result of a measure cannot be determined without context.
To evaluate a measure requires a reporting client application that can provide the context needed to retrieve the data relevant to each cell and then evaluate the expression for each cell.
That client might be an Excel PivotTable or PivotChart, a Power BI report, or a table expression in a DAX query in SQL Server Management Studio (SSMS).
Regardless of the client, a separate query is run for each cell in the results. That is to say, each combination of row and column headers in a PivotTable, or each selection of slicers and filters in a Power BI report, generates a different subset of data over which the measure is calculated
Total Sales = SUM([Sales Amount])
When a user places the TotalSales measure in a report, and then places the Product Category column from a Product table into Filters, the sum of Sales Amount is calculated and displayed for each product category.
Unlike calculated columns, the syntax for a measure includes the measure’s name preceding the formula.
In the example just provided, the name Total Sales appears preceding the formula. After you’ve created a measure, the name and its definition appear in the reporting client application Fields list, and depending on perspectives and roles is available to all users of the model.
Download the Data Analysis Expressions (DAX) Reference Guide