Top 25 ETL Testing Interview Questions with Answers tailored for experienced professionals. These questions and answers are designed to be clear, concise, and easy to understand, helping you prepare effectively for your interview:
1. What is ETL Testing?
Answer:
ETL Testing involves validating the Extract, Transform, Load process to ensure data is accurately extracted from source systems, transformed correctly, and loaded into the target system without errors.
2. What are the key objectives of ETL Testing?
Answer:
- Ensure data accuracy and completeness.
- Validate data transformation rules.
- Verify data integrity and consistency.
- Check performance and scalability of ETL processes.
3. What are the different types of ETL Testing?
Answer:
- Data Completeness Testing: Ensures all data is transferred.
- Data Accuracy Testing: Verifies data is transformed correctly.
- Data Transformation Testing: Validates business rules and logic.
- Performance Testing: Checks ETL process speed and efficiency.
- Data Integrity Testing: Ensures relationships between tables are maintained.
4. What is a data warehouse, and why is it important in ETL Testing?
Answer:
A data warehouse is a centralized repository for storing integrated data from multiple sources. It is important in ETL Testing because it ensures data is correctly loaded and available for analysis.
5. What is the difference between ETL Testing and Database Testing?
Answer:
- ETL Testing: Focuses on data migration, transformation, and loading processes.
- Database Testing: Focuses on data integrity, schema, and CRUD operations.
6. What is a staging area, and why is it used in ETL Testing?
Answer:
A staging area is a temporary storage area where raw data is held before transformation. It is used in ETL Testing to:
- Validate raw data before transformation.
- Simplify error handling and debugging.
7. What is data reconciliation in ETL Testing?
Answer:
Data reconciliation involves comparing source and target data to ensure they match after the ETL process. It ensures no data loss or corruption during migration.
8. What is a surrogate key, and how is it tested?
Answer:
A surrogate key is a unique identifier added to a table in a data warehouse. It is tested by:
- Verifying its uniqueness.
- Ensuring it is correctly mapped to natural keys.
9. What is a slowly changing dimension (SCD), and how do you test it?
Answer:
SCD is a technique to manage changes in dimension tables over time. Testing involves:
- Verifying Type 1 (overwrite), Type 2 (add new row), or Type 3 (add new column) updates.
- Ensuring historical data is preserved (if applicable).
10. What is data profiling, and why is it important in ETL Testing?
Answer:
Data profiling involves analyzing source data to understand its structure, quality, and relationships. It is important in ETL Testing to:
- Identify data anomalies.
- Design effective test cases.
11. What is the difference between incremental load and full load in ETL Testing?
Answer:
- Full Load: All data is extracted and loaded into the target system.
- Incremental Load: Only new or updated data is loaded.
Testing involves verifying data accuracy and completeness for both scenarios.
12. What is a lookup transformation, and how do you test it?
Answer:
A lookup transformation compares source data with reference data to retrieve additional information. Testing involves:
- Verifying correct lookup logic.
- Ensuring accurate mapping of source and reference data.
13. What is data cleansing, and how is it tested?
Answer:
Data cleansing involves identifying and correcting errors in data. Testing involves:
- Verifying invalid data is removed or corrected.
- Ensuring data meets quality standards.
14. What is CDC (Change Data Capture), and how do you test it?
Answer:
CDC is a technique to identify and capture changes in source data. Testing involves:
- Verifying new, updated, and deleted records are captured.
- Ensuring changes are correctly applied to the target system.
15. What is a fact table and a dimension table, and how are they tested?
Answer:
- Fact Table: Contains measurable data (e.g., sales).
- Dimension Table: Contains descriptive data (e.g., product).
Testing involves verifying data accuracy, relationships, and integrity between these tables.
16. What is a star schema, and how is it tested?
Answer:
A star schema is a data warehouse design with a central fact table connected to dimension tables. Testing involves:
- Verifying relationships between tables.
- Ensuring data is correctly aggregated.
17. What is a snowflake schema, and how is it tested?
Answer:
A snowflake schema is a normalized version of a star schema. Testing involves:
- Verifying normalized tables and relationships.
- Ensuring data integrity and consistency.
18. What is data migration testing?
Answer:
Data migration testing ensures data is accurately transferred from an old system to a new system. It involves:
- Verifying data completeness and accuracy.
- Ensuring no data loss or corruption.
19. What is performance testing in ETL?
Answer:
Performance testing checks the speed and efficiency of ETL processes. It involves:
- Testing large data volumes.
- Identifying bottlenecks and optimizing processes.
20. What is data integrity testing?
Answer:
Data integrity testing ensures data relationships and constraints (e.g., primary keys, foreign keys) are maintained in the target system.
21. What is error handling in ETL Testing?
Answer:
Error handling involves identifying and resolving errors during the ETL process. Testing includes:
- Verifying error logs and notifications.
- Ensuring failed records are reprocessed correctly.
22. What is metadata testing?
Answer:
Metadata testing involves validating metadata (e.g., table structures, data types) to ensure it matches the expected design.
23. What is data validation testing?
Answer:
Data validation testing ensures data meets business rules and requirements. It involves:
- Verifying data accuracy and completeness.
- Checking for data anomalies.
24. What is regression testing in ETL?
Answer:
Regression testing ensures new changes to the ETL process do not break existing functionality. It involves retesting previously validated scenarios.
25. What tools are commonly used for ETL Testing?
Answer:
Common ETL Testing tools include:
- Informatica Data Validation Option (DVO)
- QuerySurge
- ICEDQ
- Talend
- SQL Server Integration Services (SSIS)
These questions and answers cover the most critical aspects of ETL Testing and are designed to help you confidently tackle your interview.
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