ETL Testing Interview Questions

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If you are a fresher or a professional and looking for a platform to prepare for job interviews in ETL Testing, then this is the right place for you. The ETL Testing Interview Questions blog is designed by experts to assist you in moving ahead in your ETL Testing career without any difficulty. It guides you through the basic-to-advanced level questions that are generally asked during job interviews in this field. Go through the ETL interview questions listed below and make sure to prepare them before going for your job interview.

Table of content

1. Basic ETL Testing Interview Questions
2. Advanced ETL Testing Interview Questions
3. ETL Testing Interview Questions on SQL Queries

Most Frequently Asked Etl Testing Interview Questions

1. Compare ETL Testing with Manual Testing.
2. What is ETL?
3. Why ETL Testing is required?
4. What are the responsibilities of an ETL Tester?
5. What are the various tools used in ETL?
6. Define ETL Processing.
7. What do ETL Testing operations include?
8. List a few ETL bugs.
9. What is Fact? What are the types of Facts?
10. What are Cubes and OLAP Cubes?

Basic ETL Testing Interview Questions

1. Compare ETL Testing with Manual Testing.

2. What is ETL?

ETL refers to the Extracting, Transforming, and Loading of data from any outside system to the required place. These are the basic three steps in the data integration process.

‘Extracting’ means locating data and removing it from the source file; ‘Transforming’ is the process of transporting it to the required target file, and in the ‘Loading’ stage the file is loaded to the target system in the specified format.

3. Why ETL Testing is required?

• To keep an eye on data that is being transferred from one system to another
• To keep track of the efficiency and speed of the process
• To achieve fast and the best results

4. What are the responsibilities of an ETL Tester?

An ETL Tester:

• Requires in-depth knowledge of the ETL tools and processes
• Needs to write SQL queries for various scenarios during the testing phase
• Should be able to carry out different types of tests and keep a check on the other functionalities of the process
• Needs to carry out quality checks on a regular basis

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5. What are the various tools used in ETL?

• Cognos Decision Stream
• Oracle Warehouse Builder
• Business Objects XI
• SAS Business Warehouse
• SAS Enterprise ETL Server

Advanced ETL Testing Interview Questions

6. Define ETL Processing.

ETL Testing Process:
Although there are many ETL tools, there is a simple testing process commonly used in ETL Testing. It is as important as the implementation of the ETL tool into your business. Having a well-defined ETL Testing strategy can make the testing process much easier. Hence, this process needs to be completed before you start the data integration with the selected ETL tool.

In this ETL Testing process, a group of experts comprising the programming and developing team will start writing SQL statements. The development team may customize them according to the requirements.

ETL Testing process has the following stages:

• Analyzing requirements: Understanding the business structure and their particular requirements.
• Validation and test estimation: Estimating the time and expertise required to carry on with the procedure.
• Test planning and designing the testing environment: Based on the inputs from the estimation, an ETL environment is planned and worked out.
• Test data preparation and execution: Data for the test is prepared and executed as per the requirements.
• Summary report:  Upon the completion of the test run, a brief summary report is prepared for improvising and concluding.

7. What do ETL Testing operations include?

ETL Testing includes:

• Verifying whether the data is transformed accurately according to business requirements
• Verifying that the projected data is loaded into the data warehouse without any truncation or data loss
• Making sure that the ETL application reports any invalid data and replaces with default values
• Making sure that the data loads within the expected time frame to improve scalability and performance

8. List a few ETL bugs.

• Calculation Bug
• User Interface Bug
• Source Bug
• Load Condition Bug
• ECP-related Bug

In addition to the above ETL Testing questions, there may be other vital questions where you might be asked to mention the ETL tools that you have used earlier. Also, you might be asked about any debugging issues you have faced in your earlier real-time experience.

9. What is Fact? What are the types of Facts?

Fact is a central component of a multi-dimensional model that contains the measures to be analyzed. Facts are related to dimensions.

Types of facts are:

• Additive Facts
• Semi-additive Facts
• Non-additive Facts

10. What are Cubes and OLAP Cubes?

Cubes are data processing units comprised of fact tables and dimensions from the data warehouse. They provide a multi-dimensional analysis.

OLAP stands for ‘Online Analytics Processing,’ and OLAP Cubes store voluminous data in a multi-dimensional form for reporting purposes. They consist of facts called ‘measures’ categorized by dimensions.

11. Mention the types of Data Warehouse applications. What is the difference between Data Mining and Data Warehousing?

Types of data warehouse applications are:

• Info Processing
• Analytical Processing
• Data Mining

Data mining can be defined as the process of extracting hidden predictive information from large databases and interpreting the data, while data warehousing may make use of a data mine for the analytical processing of the data in a faster way. Data warehousing is the process of aggregating data from multiple sources into one common repository.

ETL Testing Interview Questions on SQL Queries

12. What is the difference between INNER JOIN and LEFT JOIN in SQL?

• INNER JOIN returns rows that have matching values in both tables.
• LEFT JOIN returns all rows from the left table and the matched rows from the right table. If there is no match, NULL is returned for the right table.

13. What is the purpose of the SQL DISTINCT keyword?

The DISTINCT keyword is used to eliminate duplicate values from the result set, ensuring that unique records are returned.

14. What are aggregate functions in SQL? Name a few.

Aggregate functions perform a calculation on a set of values and return a single value. Common aggregate functions are:

• COUNT()
• SUM()
• AVG()
• MAX()
• MIN()

15. What will happen if we use GROUP BY without any aggregate function?

Using GROUP BY without an aggregate function will group the data, but the result will show unique values based on the columns specified, without performing any aggregation. Typically, an error may occur if no meaningful group is returned.

16. What is the difference between HAVING and WHERE clauses in SQL?

• WHERE is used to filter rows before any grouping is done.
• HAVING is used to filter groups after the aggregation is performed.
• WHERE filters individual rows, while HAVING filters the results of a GROUP BY.

17. Explain the purpose of the UNION and UNION ALL operators.

• UNION combines the results of two queries and removes duplicates.
• UNION ALL combines the results of two queries without removing duplicates.

18. What is a subquery and when is it used in SQL?

A subquery is a query nested within another query. It is used to retrieve data that will be used in the main query. Subqueries can be used in the WHERE, FROM, and SELECT clauses.

19. What is the difference between a primary key and a unique key in SQL?

• A primary key uniquely identifies each record in a table and does not allow NULL values.
• A unique key also enforces uniqueness but allows NULL values (one NULL per column).

20. What is the use of the SQL "BETWEEN" operator?

The BETWEEN operator is used to filter the results within a certain range, such as dates or numbers. The range is inclusive (it includes the boundary values).

21. What is an index in SQL, and why is it important?

An index is a database object that improves the speed of data retrieval operations on a table. It is important because it allows faster searches, especially in large tables, by providing a quick lookup method.

22. Explain what is a "JOIN" and the different types of JOINs available in SQL.

A JOIN combines rows from two or more tables based on a related column between them. The types of JOINs are:

• INNER JOIN: Returns records with matching values in both tables.
• LEFT JOIN: Returns all records from the left table and matches records from the right.
• RIGHT JOIN: Returns all records from the right table and matches records from the left.
• FULL JOIN: Returns all records when there is a match in either the left or right table.
• CROSS JOIN: Returns the Cartesian product of both tables.

23. How does SQL handle NULL values?

SQL treats NULL as an unknown value. Comparisons with NULL using = or != return unknown. You must use IS NULL or IS NOT NULL to check for NULL values.

24. What is a self-join in SQL?

A self-join is a join where a table is joined with itself. It is useful for comparing rows within the same table, typically when there is a hierarchical relationship in the data.

25. What is the difference between TRUNCATE and DELETE commands in SQL?

• TRUNCATE removes all rows from a table but does not log individual row deletions.
• DELETE removes rows one by one and logs each row deletion, allowing for rollback.
• TRUNCATE is faster and does not generate individual row delete logs, while DELETE can be rolled back and has more overhead.