In DAX, **context transition** refers to how row context can transition to filter context when a calculated column or measure is evaluated. Understanding this concept is essential for working with DAX in Power BI because context transition affects how data is filtered and evaluated in different scenarios, particularly when using functions like `CALCULATE`

.

**Breaking Down Context Types**

**Row Context**: Applies when DAX evaluates a row in a calculated column or when iterating through each row in a table (e.g., using functions like`SUMX`

or`FILTER`

).**Filter Context**: Refers to the filters that are applied to a calculation, such as report-level filters or filters applied in a visual.

**Context Transition occurs when a row context is converted into a filter context**—a change in how DAX interprets the data and calculations. This transition primarily happens when you use functions like `CALCULATE`

, which allows row-by-row evaluations to act as filters, applying these filters to the entire calculation.

**Example: Demonstrating Context Transition with **`CALCULATE`

`CALCULATE`

Let’s say you have a `Sales`

table with columns for `ProductID`

, `SalesAmount`

, and `Quantity`

. You also have a `Products`

table with columns `ProductID`

and `ProductCategory`

.

Now, imagine you want to create a calculated column in the `Sales`

table that sums up all sales in the same product category as the current row.

**Without Context Transition (Incorrect Approach):**

This formula won’t work correctly because the row context of each `ProductCategory`

is not transitioned to filter context across the entire `Sales`

table.

**With Context Transition (Correct Approach):**

To get the correct result, you need to use `CALCULATE`

to apply context transition:

In this formula:

`CALCULATE`

transitions the row context of the current row’s`ProductCategory`

to filter context.- The
`ALLEXCEPT`

function removes any existing filters except for the`ProductCategory`

, so the calculation sums up all rows in the same category, ignoring other filters in the`Sales`

table.

**Why is Context Transition Important?**

**Essential for Complex Measures**: Many complex DAX measures rely on context transition, especially those that require evaluating data dynamically based on row-by-row conditions.**Enables Row Context to Act as Filter Context**: Context transition allows DAX functions that rely on filter context, like`CALCULATE`

, to apply row-level logic more broadly.

**Key Takeaway**: Always remember that `CALCULATE`

is the primary function that enables context transition. Whenever you’re dealing with row-by-row calculations that should also filter across the table, consider if context transition is necessary to get accurate results.

**Interview Tip**: Explaining context transition with an example is a great way to demonstrate understanding. Highlighting how `CALCULATE`

and `ALLEXCEPT`

work together to manage filter context can show your grasp of advanced DAX principles.