In Power BI, a context transition refers to the process of switching between filter contexts and row contexts in a DAX calculation. Context transitions are important to understand because they can affect the way your DAX calculations are evaluated and the results they produce.
Here's a brief overview of the two types of contexts in Power BI:
Filter context: This is the set of filters that are currently applied to the report or visual, either by the user or by the system. The filter context determines which rows of data are visible and which values are included in any calculations.
Row context: This is the context that is created by the current row of data being evaluated in a DAX calculation. The row context defines the current value of each column in the current row, and is used to calculate any measures or expressions that depend on those values.
Context transitions occur when a DAX calculation needs to switch between the filter context and row context, or vice versa. For example, when you use a CALCULATE function in a measure, you are creating a new filter context that is based on the existing filter context. Any filters that you specify in the CALCULATE function will be applied on top of the existing filter context.
When you use a DAX function that iterates over the rows of a table, such as SUMX or AVERAGEX, you are creating a new row context for each row of data that is being evaluated. The DAX expression inside the iteration function is evaluated in the context of each individual row, and the results are aggregated to produce the final result.
Context transitions can be complex, especially when you have multiple filters, relationships, and calculations in your report. It's important to understand how context transitions work and how they affect your calculations, so that you can create accurate and meaningful reports in Power BI.
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