Телеграм канал «Data Analytics»

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9️⃣ What is DIVIDE and why would you use it instead of "/"?

Sample Answer:
DIVIDE is a DAX function used for division and allows us to safely handle situations where the denominator is zero or blank.

For example:
Profit Margin =
DIVIDE(
[Total Profit],
[Total Sales],
0
)

If Total Sales is zero, the expression returns 0 instead of producing a division-by-zero error.

This makes DIVIDE a safer choice for business calculations.

🔟 How would you explain context transition in an interview?

Sample Answer:

Context transition happens when CALCULATE converts the current row context into filter context.

It is particularly important when CALCULATE is used inside an iterator or when working with calculated columns.

For example, if I use CALCULATE inside an iterator such as SUMX, the current row can influence the filter context used by the calculation.

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📊 Data Analyst Interview Series — Part 14

In the previous part, we covered Power BI fundamentals. Now let's focus on one of the most important areas in Power BI interviews:

📊 DAX & Context in Power BI

1️⃣ What is DAX and why is it important?

Sample Answer:

DAX stands for Data Analysis Expressions. It is the formula language used in Power BI to create measures, calculated columns, and calculated tables.

DAX is important because it allows us to create business calculations that go beyond simple aggregations.

For example:

Total Sales =
SUM(Sales[SalesAmount])

I can then use this measure in different visuals and Power BI will automatically recalculate it based on filters and selections.

2️⃣ What is filter context in DAX?

Sample Answer:

Filter context refers to the filters that affect how a DAX calculation is evaluated.

For example, if I have a Total Sales measure and select:

Region = South

Year = 2026

Power BI calculates Total Sales only for records matching those filters.

This is why the same measure can return different results depending on the filters applied to a report.

3️⃣ What is row context?

Sample Answer:

Row context means that DAX evaluates an expression for the current row.

It is commonly encountered when working with calculated columns.

For example:

Profit =
Sales[Revenue] - Sales[Cost]

Power BI evaluates this calculation separately for each row in the Sales table.

A simple way to remember:

Row context → Current row

Filter context → Current set of filtered data

4️⃣ What is the difference between a measure and a calculated column?

Sample Answer:

A calculated column is calculated for every row and stored in the model, while a measure is calculated dynamically when it is used in a visual.

For example:

Calculated column:

Profit =
Sales[Revenue] - Sales[Cost]

Measure:

Total Profit =
SUM(Sales[Profit])

For aggregations and KPIs, I generally prefer measures because they respond dynamically to filters.

5️⃣ What is CALCULATE in DAX?

Sample Answer:

CALCULATE is one of the most important DAX functions because it evaluates an expression after modifying the filter context.

For example:

Sales for India =
CALCULATE(
[Total Sales],
Customers[Country] = "India"
)

This calculates Total Sales specifically for customers from India.

CALCULATE is heavily used in advanced DAX calculations.

6️⃣ What is the difference between SUM and SUMX?

Sample Answer:

SUM simply adds values from a single column.

Example:

Total Sales =
SUM(Sales[SalesAmount])

SUMX is an iterator. It evaluates an expression row by row and then adds the results.

Example:

Total Revenue =
SUMX(
Sales,
Sales[Quantity] * Sales[UnitPrice]
)

I would use SUM when the required values already exist in a column, and SUMX when I need to perform a calculation for each row before aggregating.

7️⃣ What is the difference between ALL and ALLSELECTED?

Sample Answer:

ALL removes filters from the specified table or column.

For example, it can be used to calculate a value against the overall total.

ALLSELECTED generally preserves the filters selected by the user while removing some visual-level filtering.

This is particularly useful when calculating percentages or rankings within the user's selected report context.

For example, if a user selects 2026, ALLSELECTED can help calculate values relative to the selected 2026 data rather than ignoring the user's selection completely.

8️⃣ How do you calculate percentage of total sales in Power BI?

Sample Answer:

First, I create a Total Sales measure:

Total Sales =
SUM(Sales[SalesAmount])

Then I can calculate percentage of total:

Sales % =
DIVIDE(
[Total Sales],
CALCULATE(
[Total Sales],
ALL(Sales)
),
0
)

This allows me to compare the current filtered sales against the overall sales total.

9️⃣ What is the difference between a calculated column and a measure?

Sample Answer:
A few DAX formulas in your example need syntax corrections. Here’s a polished version:

A calculated column is evaluated row by row and stored in the model. A measure is evaluated dynamically based on the report’s filter context.

For example, a calculated column could be:
Profit = Sales[Revenue] - Sales[Cost]
A measure could be:
Total Sales = SUM(Sales[SalesAmount])
I generally prefer measures for aggregations and KPI calculations because they respond dynamically to report filters.

🔟 What is DAX in Power BI?

Sample Answer:

DAX stands for Data Analysis Expressions. It is the formula language used in Power BI to create measures, calculated columns, and calculated tables.

For example:
Total Sales = SUM(Sales[SalesAmount])
Another example:
Profit Margin =
DIVIDE([Total Profit], [Total Sales], 0)
DAX is especially useful for business calculations such as KPIs, percentages, rankings, time intelligence, and comparisons.

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📊 Data Analyst Interview Series — Part 13

📊 Power BI Interview Questions

Part 13 — Power BI Fundamentals

1️⃣ What is Power BI and why is it used?

Sample Answer:

Power BI is a business intelligence and data visualization tool from Microsoft. It helps analysts connect to different data sources, clean and transform data, create data models, build interactive reports and dashboards, and share insights with stakeholders.

For example, I can connect Power BI to SQL Server, Excel, or CSV files, transform the data using Power Query, create a data model, write DAX measures, and build an interactive sales dashboard.

2️⃣ What are the main components of Power BI?

Sample Answer:

The major components I commonly work with are:

• Power BI Desktop — Build reports and data models.
• Power BI Service — Publish, share, and manage reports online.
• Power BI Mobile — View reports on mobile devices.
• Power Query — Connect, clean, and transform data.
• DAX — Create calculations and measures.
• On-premises Data Gateway — Connect Power BI Service to on-premises data sources.

Together, these components support the complete reporting and analytics process.

3️⃣ What is the difference between Power BI Desktop and Power BI Service?

Sample Answer:

Power BI Desktop is mainly used for developing reports and data models, while Power BI Service is a cloud-based platform used to publish, share, collaborate, and manage those reports.

For example:

Power BI Desktop → Build the report → Publish → Power BI Service → Share with users.

4️⃣ What is Power Query in Power BI?

Sample Answer:

Power Query is used for extracting, cleaning, and transforming data before loading it into the Power BI model.

For example, I can use Power Query to:

• Remove duplicates
• Handle missing values
• Change data types
• Split columns
• Merge datasets
• Append datasets
• Filter records
• Create calculated transformation columns

Power Query is essentially used for the ETL part of the process.

5️⃣ What is the difference between Power Query and DAX?

Sample Answer:

Power Query is primarily used to prepare and transform data before it enters the model, whereas DAX is used to create calculations within the Power BI model.

For example:

Power Query → Remove duplicates, merge tables, clean customer names.

DAX → Calculate Total Sales, Profit Margin, Year-over-Year Growth.

A simple way to remember it is:

Power Query = Data preparation

DAX = Data analysis and calculations

6️⃣ What is a data model in Power BI?

Sample Answer:

A data model defines how different tables are structured and connected within Power BI.

For example, I might have:

Sales
Customers
Products
Date

The Sales table can be connected to Customers, Products, and Date using relationships.

A well-designed data model makes reporting more accurate, easier to maintain, and more efficient.

7️⃣ What is a relationship in Power BI?

Sample Answer:

A relationship connects two tables using a common column.

For example:

Customers
↓
Sales[CustomerID]

This allows Power BI to understand which sales transactions belong to which customers.

Common relationship types include:

• One-to-many
• Many-to-one
• One-to-one
• Many-to-many

In most analytical models, one-to-many relationships are commonly used.

8️⃣ What is a star schema and why is it important?

Sample Answer:

A star schema is a data modeling approach where a central fact table is connected to multiple dimension tables.

For example:

FactSales
→ DimCustomer
→ DimProduct
→ DimDate
→ DimRegion

The fact table contains measurable business transactions, while dimension tables provide descriptive information.

I prefer a star schema because it makes the model easier to understand, improves filtering behavior, and generally supports better reporting performance.

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