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Data Warehousing Real-Time Interview Questions — Asked in Infosys, TCS & Capgemini (2025 Edition)

 
Interview Questions & Answers
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If you’re preparing for a Data Warehousing or ETL interview at Infosys, TCS, or Capgemini, you must be ready for real-world data modeling and performance tuning questions — not just theory.

These 2025 interview questions cover the practical side of data warehouse design, ETL workflows, fact/dimension tables, and star-schema implementations commonly used in enterprise analytics projects.

🔹 Top Data Warehousing Real-Time Interview Questions (2025 Edition)

1️⃣ What is a Data Warehouse and how is it different from an OLTP system?

Answer:
A Data Warehouse (DW) is a centralized system for storing historical and analytical data, optimized for reporting.

FeatureOLTPData Warehouse (OLAP)
PurposeDay-to-day transactionsLong-term analytics
Data VolumeSmallVery large
OperationsInsert/Update/DeleteSelect/Aggregate
ExampleBanking appSales performance dashboard

✅ Common starting question in TCS interviews.

2️⃣ Explain the difference between Star Schema and Snowflake Schema.

  • Star Schema:

    • One fact table linked to multiple dimension tables.

    • Simpler, faster queries.

  • Snowflake Schema:

    • Dimensions normalized into sub-dimensions.

    • Reduces redundancy, but increases joins.
      ✅ Capgemini often asks which schema you used in your project.

3️⃣ What are Fact and Dimension Tables? Give examples.

  • Fact Table: Stores quantitative data (e.g., sales amount, quantity).

  • Dimension Table: Stores descriptive attributes (e.g., customer, product, region).
    Example:

  • Fact_Sales: Date_Key, Product_Key, Sales_Amount

  • Dim_Product: Product_Key, Category, Brand

4️⃣ What are Slowly Changing Dimensions (SCD)?

SCDs handle historical changes in dimension data.
Types:

  • Type 1: Overwrite old data (no history)

  • Type 2: Add new record (keeps full history)

  • Type 3: Add new column (partial history)

Example (Type 2):

 
UPDATE dim_customer
SET end_date = SYSDATE
WHERE customer_id = 101 AND end_date IS NULL;

✅ Real project question from Infosys DW team.

5️⃣ What is a Data Mart and how is it related to a Data Warehouse?

A Data Mart is a subset of a Data Warehouse — focused on a specific business function.
Example:

  • Finance Data Mart

  • Sales Data Mart
    ✅ TCS often checks if you understand data segregation for analytics.

6️⃣ How do you load data incrementally in a Data Warehouse?

Incremental load = only new or changed data since last refresh.
Techniques:

  • Using Last Updated Date columns

  • Change Data Capture (CDC)

  • Comparing hash keys
    Example:

 
SELECT * FROM orders
WHERE last_updated > TO_DATE('2025-01-01', 'YYYY-MM-DD');

7️⃣ What is ETL and what tools are commonly used?

ETL: Extract → Transform → Load.
Popular tools:

  • Informatica PowerCenter

  • Talend

  • SSIS

  • Oracle Data Integrator (ODI)

  • Azure Data Factory / AWS Glue

✅ Capgemini focuses on data orchestration and automation concepts.

8️⃣ What is Data Staging and why is it used?

The staging area temporarily stores raw extracted data before transformation.
Advantages:

  • Reduces load on source systems.

  • Supports incremental data validation.

  • Simplifies error correction during ETL.

9️⃣ What are Aggregate Tables and why are they used?

Aggregate tables store pre-calculated summaries (e.g., monthly sales totals).
They:

  • Improve query performance.

  • Reduce computation on live queries.
    Example:

 
CREATE TABLE monthly_sales AS
SELECT product_id, SUM(amount) AS total_sales
FROM sales
GROUP BY product_id;

✅ Infosys uses this example in Power BI + Oracle DW interviews.

🔟 How do you ensure data quality in ETL processes?

  • Validate source vs. target counts.

  • Check for duplicates and nulls.

  • Maintain audit logs.

  • Use error handling tables for rejected records.

📌 Practical question in real-time DW project interviews.

💡 Pro Tip:

When explaining a project scenario, always describe the data flow:

“We extracted sales data from SAP, transformed it in Informatica, and loaded it into an Oracle Data Warehouse for Power BI dashboards.”

It adds credibility and aligns with real enterprise environments.

🧩 Conclusion

These Data Warehousing real-time interview questions (2025 Edition) reflect what top companies like Infosys, TCS, and Capgemini look for in ETL and BI professionals.

Focus on understanding data modeling concepts, SCD types, and incremental loading techniques — they form the foundation of every data warehouse project.

Consistent practice with ETL tools and SQL-based transformations will help you confidently answer any scenario-based question.


Topic Tags
Data Warehousing Intervie TCS Data Warehouse Inter Infosys ETL Real-Time Sc Capgemini Data Modeling Data Mart and Fact Table

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Topic Tags:  Data Warehousing Intervie (1) , TCS Data Warehouse Inter (1) , Infosys ETL Real-Time Sc (1) , Capgemini Data Modeling (1) , Data Mart and Fact Table (1) ,
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