DAX (Data Analysis Expressions) is a formula language used in Microsoft Power BI, Analysis Services, and Excel Power Pivot. It is designed for defining custom calculations, measures, and aggregations over data models.

DAX is particularly powerful for:

• Time intelligence calculations

• Custom KPIs

• Complex aggregations

• Conditional logic

Examples of DAX calculations include:

• Year-to-date revenue

• Rolling averages

• Prior period comparisons

• Conditional metrics

Unlike SQL, DAX operates on columnar, in-memory models and uses context, row context, and filter context, to calculate results dynamically.

Understanding context is the most important part of mastering DAX. Filter context changes based on slicers, visuals, and filters applied in a report.

DAX enables business logic to live inside the BI model, reducing dependency on the data warehouse. However, poorly written DAX can cause performance issues and make models hard to maintain.

Best practice is to keep DAX focused on presentation and calculations while handling heavy transformations upstream.

DAX remains a core skill for Power BI professionals building advanced analytics solutions.