07 DAX Fundamentals

5️⃣ DAX (Data Analysis Expressions) – Core Skill

DAX is the formula language of Power BI, Power Pivot, and Analysis Services Tabular. It is similar to Excel formulas but much more powerful because it works on tables and columns in a columnar in-memory database.

DAX is what turns your clean data model into actionable insights. Almost every interview for Power BI roles heavily tests DAX.

Macro: DAX Fundamentals

1. Calculated Columns vs Measures

Aspect	Calculated Column	Measure
Storage	Stored in the model (consumes RAM)	Calculated on the fly (no storage)
Context	Row Context (mainly)	Filter Context (mainly)
When to Use	When you need a value per row for slicing	For aggregations and dynamic calculations
Performance	Slower model, higher memory	Better performance
Row Level	Yes	No (unless iterated)
Examples	Profit = Sales - Cost, Full Name = First + Last	Total Sales, YoY Growth %, % of Grand Total

Golden Rule:

Use Measures whenever possible.
Use Calculated Columns only when:
- You need the value for slicing/filtering in visuals.
- You need it as a filter in relationships.
- For complex row-level logic that cannot be done in Power Query.

Common Mistake: Creating calculated columns for simple aggregations (e.g., Total Sales column).

2. Basic DAX Syntax

Measure Name = DAX Expression

Measure names can have spaces.
Columns are referenced as Table[Column].
Measures are referenced by name only (no table prefix usually).
Comments: // Single line or /* Multi-line */

3. Row Context vs Filter Context (Evaluation Context)

This is the most important and confusing concept in DAX.

Row Context: