Context

Analysts moving from SQL to Power BI are often expected to explain how DAX works at a practical level. Interviewers usually want to know whether you understand how DAX differs from standard SQL aggregation logic.

Question

Explain how to use basic DAX in Power BI. Your answer should cover:

1. What DAX is and where it is used in Power BI
2. The difference between measures and calculated columns
3. How basic DAX functions such as SUM, COUNT, DIVIDE, and CALCULATE are used
4. How DAX evaluation context differs from a typical SQL query with GROUP BY

Scope Guidance

Keep the explanation at an interview level: define the core concepts clearly, show a few short examples, and connect them to SQL concepts the interviewer already knows. You do not need advanced time intelligence or complex filter propagation details, but you should be precise about when a measure recalculates versus when a calculated column is stored.

DAX Purpose

DAX, or Data Analysis Expressions, is the formula language used in Power BI for calculations in measures, calculated columns, and calculated tables. It is designed for analytical modeling and interactive reporting rather than row-by-row transactional querying like SQL.

Measures

A measure is calculated at query time based on the current filter context in a report. This makes measures dynamic: the same measure can return different results depending on slicers, visual filters, or grouping fields.

SELECT region, SUM(sales_amount) AS total_sales
FROM sales
GROUP BY region;

Calculated Columns

A calculated column is computed once during data refresh and stored in the model. It is useful for row-level derived values, similar to adding a derived expression in a SQL SELECT or materializing a transformed field in a table.

SELECT sales_amount, quantity, sales_amount / NULLIF(quantity, 0) AS unit_price
FROM sales;

CALCULATE and Filter Context

CALCULATE is one of the most important DAX functions because it changes filter context before evaluating an expression. It is commonly used to create filtered aggregations such as sales for a specific category, year, or status.

SELECT SUM(sales_amount)
FROM sales
WHERE category = 'Electronics';

DAX vs SQL Aggregation

SQL usually defines grouping explicitly with GROUP BY, while DAX often relies on the visual and report context to determine the aggregation grain. This means a DAX measure can behave like many SQL queries depending on how the report is sliced.

SELECT product_category, SUM(sales_amount)
FROM sales
GROUP BY product_category;