These are the ETL testing interview questions for freshers that come up again and again in India, from service companies to product firms. Each has a short answer you can say out loud, and the SQL ones include queries you should be able to write from memory.
How Fresher ETL Testing Interviews Usually Run
Large IT service companies often hire freshers in bulk and then assign ETL testing projects. Processes vary by company and year, but a typical sequence is:
| Round | What to expect | How to prepare |
|---|---|---|
| Online test | Aptitude, reasoning, verbal and sometimes basic SQL or coding | Timed aptitude practice |
| Technical interview | SQL queries, ETL and data warehouse basics, testing concepts | Questions 1–25 below; write queries by hand |
| Project / scenario discussion | "What would you do if…" questions and your project | Questions 26–30 and one practice project |
| HR interview | Background, relocation, shifts, why testing | Honest, short answers |
Company-specific questions (for example "TCS ETL testing interview questions" or "Infosys ETL testing interview questions") are mostly the same fundamentals below. Master these and you are prepared for most fresher technical rounds.
ETL Testing Basics (Q1–Q10)
1. What is ETL testing?
ETL testing verifies that data extracted from source systems, transformed by business rules and loaded into a target such as a data warehouse is complete, accurate and consistent.
2. Why is ETL testing important?
Business decisions are made from warehouse reports. Missing, duplicated or wrongly transformed data leads to wrong numbers, so testers prove the data is right before users see it.
3. What are the main types of ETL testing?
Source-to-target count testing, data completeness (missing/extra records), transformation testing, data quality (nulls, duplicates, formats), metadata testing, incremental load testing, regression testing and report testing.
4. What is a source-to-target mapping document?
A document listing each target column, its source column(s) and the transformation rule. ETL test cases are written from it.
5. What is the difference between ETL testing and database testing?
Database testing checks one database (constraints, procedures, application data). ETL testing checks data moving between systems and its transformations. See ETL vs database testing.
6. What is a staging area?
A temporary area where raw extracted data is stored before it is cleaned and transformed, so source systems are not affected.
7. What is the difference between full load and incremental load?
A full load reloads all data each time. An incremental load only loads records that are new or changed since the last run.
8. What is data cleansing?
Fixing or removing incorrect, incomplete or badly formatted data, for example trimming spaces, standardising dates or removing duplicates.
9. What tools are used in ETL testing?
SQL clients (SQL Developer, SSMS, DBeaver), ETL tools such as Informatica, SSIS or Azure Data Factory, test management tools such as Jira, and Python or tools like QuerySurge for automation. See ETL testing tools.
10. What is the ETL testing process?
Requirement analysis, mapping review, test planning, test design, test data and environment setup, execution, defect logging and retest, then regression and sign-off. See the ETL testing process.
SQL Questions for Freshers (Q11–Q20)
11. How do you check record counts between source and target?
Run SELECT COUNT(*) on both tables, applying the same filters (for example the load date), and compare.
12. How do you find records missing in the target?
Use EXCEPT (SQL Server, PostgreSQL) or MINUS (Oracle) on the key columns:
SELECT customer_id FROM source.customers EXCEPT SELECT customer_id FROM dw.dim_customer;
13. How do you find duplicates?
Group by the key and keep groups with more than one row:
SELECT customer_id, COUNT(*) FROM dw.dim_customer GROUP BY customer_id HAVING COUNT(*) > 1;
14. What is the difference between WHERE and HAVING?
WHERE filters rows before grouping; HAVING filters groups after GROUP BY and can use aggregates such as COUNT(*).
15. What is the difference between INNER JOIN and LEFT JOIN?
INNER JOIN returns only matching rows. LEFT JOIN returns all rows from the left table, with NULLs where there is no match, which is useful for finding orphans.
16. How do you find NULLs in a mandatory column?
SELECT COUNT(*) FROM table WHERE column IS NULL; the expected result is 0.
17. What is the difference between UNION and UNION ALL?
UNION removes duplicate rows; UNION ALL keeps them and is faster.
18. What is the difference between DELETE, TRUNCATE and DROP?
DELETE removes selected rows and can be rolled back in most databases; TRUNCATE removes all rows quickly; DROP removes the table itself.
19. How do you validate a transformation rule?
Apply the rule to the source data in SQL, then compare the result with the target column using EXCEPT/MINUS or a join on the key.
20. What is the second-highest salary query?
One common answer: SELECT MAX(salary) FROM emp WHERE salary < (SELECT MAX(salary) FROM emp). Freshers are often asked this as a general SQL check. More: SQL queries for ETL testing interviews.
Data Warehouse Questions (Q21–Q25)
21. What is a data warehouse?
A central database that stores integrated, historical data from many sources for reporting and analysis.
22. What are fact and dimension tables?
Fact tables store measures (sales amount, quantity). Dimension tables store descriptive attributes (customer, product, date).
23. What is a star schema?
A design with one fact table in the centre joined to several dimension tables, like a star.
24. What is SCD Type 1 vs Type 2?
Type 1 overwrites old values with no history. Type 2 adds a new row for each change, keeping history with effective dates or a current flag.
25. What is a surrogate key?
A system-generated key (usually a number) used in the warehouse instead of the source business key.
Project and HR Questions (Q26–Q30)
26. Why do you want to be an ETL tester?
Give an honest reason, such as enjoying SQL and data, and back it up with what you have learned and practised.
27. Explain a project you have worked on.
Freshers can explain a practice project honestly: the source data, the transformation, the checks you wrote and a defect you found. See how to explain an ETL project.
28. What would you do if source and target counts do not match?
Re-run the count with the same filters, find the missing or extra records with EXCEPT/MINUS, check rejects and error tables, and log a defect with sample keys.
29. How do you report a defect?
Summary, steps (the query), expected vs actual result, sample rows, load date or batch ID, severity and screenshots of query results.
30. Are you comfortable with SQL?
Answer honestly and prove it: mention the queries you can write (joins, GROUP BY, HAVING, EXCEPT) and offer to write one.
Tips for Freshers
- SQL decides most technical rounds. Practise writing queries on paper as well as on a laptop.
- Have one project ready that you can explain in two minutes, and be honest that it is a practice project.
- Test yourself with our free ETL testing mock test.
- Read the job description. If it lists Informatica or Python, revise the basics of that tool.
- Next steps: ETL testing jobs in India and the ETL testing resume guide.
Prepare for your ETL testing interview: top 50 questions · in-depth answers · for experienced (3–5 years) · for 10 years experience · SQL query questions · scenario-based questions · data warehouse questions · free mock test · database testing questions
More guides for Indian students: ETL full form · ETL testing with example · ETL testing jobs in India · ETL testing course in India · ETL training in Hyderabad, Bangalore & Pune · best ETL testing course on Udemy · ETL testing roadmap · ETL tester vs ETL developer · free ETL testing mock test · SQL for testers · QA to data engineer · ETL testing kya hai (Hindi)
Frequently Asked Questions
What questions are asked in an ETL testing interview for freshers?
Freshers are mostly asked ETL basics, SQL queries (counts, joins, duplicates, EXCEPT/MINUS), data warehouse concepts such as facts, dimensions and SCDs, the ETL testing process, and how they would handle a count mismatch.
Is SQL compulsory for an ETL testing fresher interview?
Yes. SQL is the main technical skill for ETL testers, and almost every fresher interview includes writing or explaining SQL queries.
Are TCS and Infosys ETL testing interview questions different?
The process differs by company, but fresher technical questions are largely the same fundamentals: SQL, ETL concepts, data warehouse basics and simple scenarios.
How should a fresher explain an ETL project?
Describe a practice project honestly: the source data, the transformation, the checks you wrote in SQL, a defect you found and how you reported it.
How can freshers practise ETL testing interview questions?
Write SQL daily, take mock tests, and use practice tests with detailed answers, such as the ETL testing interview practice tests on Udemy.
Asim Noaman Lodhi
Certified Google Partner · QA Consultant · 12+ Years IT
QA consultant specializing in ETL testing and data quality. Trained 913+ students to transition into data testing roles through hands-on, real-world instruction.