10 Common ETL Interview Questions (With Example Answers)

By Tonya Fister

Updated September 16, 2022 | Published December 12, 2019

Updated September 16, 2022

Published December 12, 2019

Tonya Fister has seven years of experience writing about a wide variety of subjects, including health, wellness and technology.

Related: Top Interview Tips: Common Questions, Body Language & More

In this video, we dissect an entire job interview from start to finish. We analyze everything from common interview questions to etiquette and how to follow up.

When interviewing for a position in the technology industry, demonstrating your technical knowledge of ETL, which stands for extract, transform and load, can help you make a positive impression on a hiring manager. Prospective employers often want someone with the expertise and skills to create procedures and functions and help create or operate their databases. Learning what questions a hiring manager may ask you in an interview for this position can help you prepare and increase your chances of receiving a job offer.

In this article, we share 10 questions that may be common in an ETL testing interview and provide sample answers to help you create yours.

10 ETL testing interview questions

Here are 10 questions you may encounter during your interview, along with why hiring managers ask them and how you can answer them most effectively:

1. What is ETL?

By asking this question, the interviewer wants to learn whether you can communicate the process in a manner that's easy to understand for people who don't work in this industry or are learning the topic. By showing that you can communicate the data integration process clearly, you're also demonstrating an ability to communicate effectively with those on your team.

Example: "ETL, which stands for extract, transform and load, is a type of data integration process. It uses three steps to combine data from multiple sources before loading it into a data warehouse. In the extraction stage, data scientists collect data from multiple sources to convert it into a usable format before loading it or writing it into a new database.

"ETL is a foundation for data analytics, such as monthly reporting expenses for businesses, but it also improves backend processes. The loaded data is in a format the end users can view and understand easily, which then helps them make business decisions."

Read more: What Is ETL and Why Is It Important? (Definition, Uses and FAQ)

2. What is the importance of ETL testing?

This question assesses your understanding of why it's essential to use the ETL process. A hiring manager might ask this question because someone who understands why it's essential to use the ETL process for data integration may be more likely to avoid any mistakes in the data integration process. In your answer, try to explain the benefits ETL testing can provide for team members and the company overall.

Example: "People working with data can't assume that, if left alone, nothing will happen to the data during the ETL process. This is why it's imperative to verify, validate and quantify to prevent data duplication and data loss. ETL testing ensures you can transform data more quickly and efficiently while preventing and identifying data duplication or data loss. ETL testing also ensures the ETL process runs smoothly and the data implementation follows client guidelines while moving the data to its final destination whole and secure."

3. What are the steps in an ETL process?

This question seeks to measure how well you understand the ETL process. Candidates who know the steps of the processes they use often can work faster and more efficiently than those who don't. In your answer, try to outline what each step is and what it involves.

Example: "There are five steps involved in the ETL process, and the team involved must perform each step sequentially. The five steps of the ETL process include extraction, cleaning, transforming, loading, and analyzing, with the most important steps being transform and load. During the extraction process, the system retrieves raw data from a data pool and moves the data to a temporary data repository. Next, the repository cleans the data from the pool to improve its quality before sending it to the transformation process.

"In this step, the tester structures and transforms the data to match the requirements of the target client. Next, you load the cleaned and transformed structured data into a data warehouse for analysis and use. Finally, the organization then analyzes the data in the warehouse to gain business insights that help the company grow and increase its revenue."

4. Can you name some common tools in ETL and why you use them?

The answer to this question lets the interviewer know how well the candidate understands which tools are vital to the ETL process and why. Try to provide specific examples in your answer, which can demonstrate you have expertise with these tools and can use them effectively.

Example: "Using ETL testing tools increases IT productivity and speeds the extracting process with big data. Using the tools correctly to extract and process data means you don't have to use traditional programming methods that can be time-consuming, costly and labor-intensive. Because technology evolves, it only makes sense that our solutions also do. That's why there are now many ways to perform ETL testing that partners well with the environment and source data. Open source ETL tools exist, such as Hadoop, Pentaho Data Integration, and Talend Open Studio."

Read more: 16 Open-Source ETL Tools for Effective Data Warehousing

5. What are initial load and full load?

These two processes have distinct purposes in the ETL process, so an interviewer may ask this question to ensure you have strong attention-to-detail skills and can explain how they differ. Consider explaining what each load type is and describing how the term terms compare.

Example: "Initial load refers to preliminary data loading and populating data from multiple sources into the data mart for the first time. With an initial load, the IT professional controls how much data they process to prevent encountering out-of-memory exceptions. Because this process needs a larger capacity, it's important to schedule downtime for when you need the initial load to take place. With initial load, you're managing transaction and history tables and adding them to data flows, so it's crucial to use load intervals and filters.

"A full load refers to when all the data records replace the old data records at once. The IT professional deletes old contents from the table and then loads the table with the new data. Full loads have a more straightforward design process that's easy to implement. For example, if you notice errors in the full load, all you have to do is rerun the loading process without having to worry about extra data cleaning."

6. What are the roles and responsibilities of an ETL tester?

Interviewers use this question to determine how well the candidate understands the job position. Being able to show you know and understand what work you'd perform if they hire you can help reassure the hiring manager you have the proper skills and qualifications to succeed.

Example: "As with many IT professions, there's a great demand for ETL testers who can extract data, validate data sources, transform data and load the data onto tables. This requires the ETL tester to have an in-depth knowledge of how to use ETL tools and processes and identify problems and come up with solutions promptly. Other responsibilities include p erforming back-end data-driven tests, c onducting quality checks, t esting primary keys, defaults and other ETL functions and c reating, reviewing and approving design specifications and requirements."

Related: ELT vs. ETL in Data Warehousing: What Are the Differences?

7. What are the three tiers in ETL?

An interviewer may ask to ensure you have at least a basic understanding of how ETL testing works and what its common components are. Even if you have little experience in the position, being able to show you have knowledge about the process can reassure the hiring manager you're a good fit. In your answer, try to explain what each layer does after you define it.

Example: "Most data warehouses have three layers, or structures. The first layer, which is the staging layer, is for data that multiple outside data sources will remove and compile. Next is the data integration layer, which helps with transforming the data to meet the needs of the end-user company. The access layer, the third layer, is a dimension layer, and it stores the information for internal use and allows users to retrieve the data they need for analysis."

8. What are some challenges of ETL testing?

The interviewer may ask this to clarify whether you have a realistic viewpoint of the difficulties you may face in the position. Showing you understand what challenges you may face allows you to explain you know how to overcome them. Explaining this in your answer can highlight your critical thinking and problem-solving skills, which are vital for this role.

Example: "It's true that even though everybody recognizes the importance of ETL testing, it doesn't mean the process isn't without its challenges. The sheer volume and nature of the data are one of the major challenges ETL testers face. Keeping in mind the volume of data, if a customer changes their requirements, it requires an ETL tester to rerun the data, or we might need to create new documents or SQL scripts.

"Other challenges include corrupt data sources or data loss during migration. I've found that if I double-check any inputs and processes during testing, I'm able to better protect the data and prevent these issues from occurring. I also keep regular logs and documentation, which I can refer to if I encounter a problem."

9. Can you explain partitioning, hash partitioning and round-robin partitioning?

Working with different types of data often requires different partition methods. Because of this, an interviewer might want to know how well you understand each type and whether you have experience using them. Using examples from your experience can be an excellent way to answer this question.

Example: "In ETL, we partition the data storage into subdivisions so we can improve performance. The types of partitions I've used are hash partitioning and round-robin partitioning. With hash partitioning, I use a hash key to create rows across the partitions, but in a round-robin partition, the server creates a balanced load by partitioning the same number of rows.

"For example, the server would assign the first row to the first partition, the second row to the second partition and so on. It then comes back and assigns row four to partition one, row five to partition two, and row six to partition three, which it continues until it finishes assigning rows."

Related: What Does an ETL Developer Do? (Plus Duties and Salary Info)

10. Can you explain the data cleaning process?

The interviewer may want to understand how you handle the detailed process of data cleaning. By explaining why it's important to clean data, you show you're dedicated to the needs of the end client and can provide them with reliable, accurate data.

Example: "When you're combining data from multiple sources, there's a strong possibility you'll encounter mislabeled or duplicate data that causes undesirable outcomes for the end users. Data cleaning removes data from the data set that's incorrect, incomplete, corrupt or duplicated, but the steps often vary depending on the dataset. Sometimes, you remove irrelevant data and fix structural errors, while other times, you filter out outliers and then validate the data."

Please note that none of the companies mentioned in this article are affiliated with Indeed.

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