General
ETL Testing Interview Questions and Answers
1. What is ETL Testing?
ETL Testing is the process of verifying, validating, and ensuring the accuracy of data during the ETL (Extract, Transform, Load) process. It involves testing data extraction, data transformation, and data loading phases to ensure data quality and consistency.
2. What are the key differences between ETL Testing and Database Testing?
- ETL Testing focuses on validating the data flow from source to destination, ensuring data transformations, and verifying data integrity.
- Database Testing involves checking constraints, indexes, and stored procedures within a database.
3. What are the challenges faced in ETL Testing?
Common challenges include data loss, data duplication, data transformation errors, managing heterogeneous data sources, and dealing with large data volumes.
4. Explain the ETL process?
The ETL process has three main phases:
- Extract: Data is extracted from various sources.
- Transform: Data is cleaned, aggregated, and converted to the required format.
- Load: Transformed data is loaded into the target data warehouse.
5. What is the importance of ETL Testing in data warehousing?
ETL Testing ensures data accuracy, validates data transformations, and guarantees the reliability of business intelligence reports generated from the data warehouse.
6. What is the ETL Testing life cycle?
The ETL Testing life cycle includes:
- Requirements analysis
- Test planning
- Test case design
- Test execution
- Defect reporting and closure
- Sign-off
7. What is data validation testing in ETL?
Data validation testing ensures that extracted data matches source data, transformation rules are applied correctly, and the final loaded data is accurate.
8. What is data mapping in ETL Testing?
Data mapping defines the transformation rules and relationships between source and target data fields. It is critical for ensuring data transformation is accurate.
9. What is Incremental load testing in ETL?
Incremental load testing verifies that only new or modified data is loaded into the target data warehouse, without affecting existing data.
10. What are the different types of ETL Testing?
Types of ETL Testing include:
- Data completeness testing
- Data accuracy testing
- Data transformation testing
- Regression testing
- Performance testing
11. Which tools are commonly used for ETL Testing?
Popular ETL Testing tools include Informatica, DataStage, Talend, Ab Initio, and Pentaho.
12. What is ETL Testing automation and which tools support it?
ETL Testing automation uses tools like Selenium, QuerySurge, and Datagaps ETL Validator to reduce manual effort and improve testing efficiency.
13. How does QuerySurge help in ETL Testing?
QuerySurge automates data validation by comparing large volumes of source and target data, ensuring data quality and faster ETL testing.
14. What is the role of SQL in ETL Testing?
SQL is extensively used in ETL Testing for data validation, data transformation checks, and writing queries to compare source and target data.
15. What are the benefits of ETL Testing Automation?
Automation increases testing speed, improves accuracy, supports testing of large data sets, and ensures repeatability of tests.
16. What is a Surrogate key and why is it important in ETL?
A surrogate key is a unique identifier used in the data warehouse to maintain relationships between tables. It is especially useful for tracking data transformations.
17. How do you handle Error logs in ETL Testing?
Error logs are analyzed to identify issues like data load failures, transformation errors, and mismatched data. Proper error handling mechanisms are implemented to resolve these issues.
18. What is Partitioning in ETL?
Partitioning divides large data sets into smaller, manageable parts to optimize performance during data extraction and loading.
19. What is a Staging area in ETL?
The staging area is an intermediate storage space where raw data is cleansed and transformed before being loaded into the data warehouse.
20. How do you perform Performance testing in ETL?
Performance testing evaluates the efficiency of the ETL process by analyzing data load times, query performance, and overall system stability under large data volumes.
21. How do you test slowly changing dimensions SCD in ETL?
Testing involves validating the correct application of SCD Type 1 (overwrite), SCD Type 2 (history tracking), or SCD Type 3 (limited history tracking) rules.
22. How do you handle Duplicate data in ETL Testing?
Duplicate data is identified and removed during the data transformation phase using deduplication techniques or SQL queries.
23. What is a checksum in ETL Testing?
A checksum is a hash value used to compare and validate data integrity between source and target systems.
24. How do you validate data transformations in ETL Testing?
Validation involves checking if the transformation logic is applied correctly by comparing input and output data.
25. How do you handle Schema changes during ETL Testing?
Schema changes are managed by updating the ETL mappings, modifying the data transformation rules, and retesting the affected components.
