ETL Interview Questions for Hiring the Perfect Resource


Your company is growing rapidly and so is the data coming from different sources. You know that a highly qualified ETL developer can help you manage and transform all your unstructured data and hence enable you to make informed decisions. There may be many developers in the market, but it’s difficult to assess and identify the perfect resource. So how do you find that perfect resource? Are you worried about the painstaking process and effort it will take to hire someone like that? What should be the key ETL interview questions that will help you assess your candidate wholly? How do you manage the technical assessment and put their skills to the test? And most importantly, how do you do all this in the least possible time and using minimum effort?

Let us help by showing you how to do a thorough assessment of your candidates while not taking up too much of your time. Our list of essential interview questions will pinpoint your candidates’ strengths and weaknesses and enable you to identify your required resource, and our initial assessment will help you make sure you have just the right number of candidates to interview.

Defining the Role and Responsibilities of an ETL Developer

In a nutshell, the responsibilities of ETL developers are exactly what their job titles imply. Extract, Transform, Load. This means that data needs to be extracted from various data sources and business processes, transformed into a format easier to understand, and then loaded into the data warehouse so the data is up-to-date. It’s also the developer’s responsibility to design a data storage system best suited for the business.

Hence, the following is a list of responsibilities of an ETL Developer:

  1. Determining data storage requirements
  2. Designing and building a data warehouse catering to the needs of the specific organization and managing the infrastructure
  3. Creating and improving data solutions to facilitate seamless data delivery
  4. Collecting, deconstructing, managing and analyzing big data
  5. Developing complex applications which can extract, transform and load data
  6. Ensuring data quality
  7. Data modeling
  8. Testing of the tools and data pipelines
  9. Using data models to format and transform data
  10. Defining data warehouse architecture
  11. Validating data flows

Skills you Should Look for in Order to Find the Perfect ETL Developer

There are certain skills you should be looking for when hiring an ETL developer, especially when you want to hire the perfect resource. Firstly, the candidate should have a Software Engineering background. Then, he should make it easier for the data teams by showing efficiency in his work. 

These are some of the key ETL skills a highly qualified ETL Developer is expected to have:

  1. Knowledge of Programming languages
  2. Knowledge of Database Development
  3. Knowledge of Data Analytics
  4. Experience with ETL tools
  5. Data Mapping
  6. SQL expertise
  7. Knowledge of a Scripting Language
  8. Creativity
  9. Troubleshooting/ Problem Solving
  10. Organization
  11. Communication skills

Assessing the ETL Candidate BEFORE the Interview

This part of the process is as important as the interview. This is because interviewing every candidate would take a lot of time, and time is money. You need to shortlist the candidates by assessing their resumes. Then you can put their skills to the test using assessments and coding tests to further shortlist those you should interview. But how do you make such assessments and tests or who will make it for you?

This is where HirinGuru comes in. HirinGuru is a machine learning-based recruitment solution which helps you in the recruitment process with technical screening, thereby leading you to the perfect candidate to interview. It minimizes hiring time, helps you look beyond the resume, and drastically improves interview-to-hire ratio. This is because HirinGuru enables you to interview only those candidates who have the required skill-level as it screens the best candidates from your pool based on how they do on their tests. Assessments and evaluations at HirinGuru include multiple-choice questions, coding tests, and even machine learning algorithm tests to better evaluate your ETL candidates. 

Now that you have the candidates with the perfect skill-set, you can move further along and interview them. 

Top ETL Interview Questions to Assess and Hire the perfect ETL Developer

In the final stage of hiring, you need a list of key ETL questions that help you determine who is the best candidate out of all and who can be the perfect resource for you. The questions you refer to will largely depend on the specific role you are looking to fill because say, ETL Architect interview questions will be similar to ETL Developer interview questions, but not role-specific. Here is an exhaustive list of questions to help you find your required resource. 

ETL Developer Interview Questions:

  1. What do you mean by snapshots and what are their features and characteristics?
  1. What are some disadvantages of using indexes?
  1. Define staging.
  1. What is the difference between full load and incremental load?
  1. How do you prepare for an incremental load?
  1. How would one update a very big table having millions of rows?
  1. What are some common transformations in ETL processes?
  1. What do you know about ETL testing and how is it different from development?
  1. What advantage do third party tools like SSIS have as opposed to SQL scripts?
  1. What is the main role of a data-mining engine in a data mining system?
  1. How can we fine-tune mapping?
  1. What are the main differences between connected and unconnected lookup?
  1. Explain the difference between hash partitioning and round-robin partitioning?
  1. Describe a materialized view log?
  1. If there are hundreds and thousands of records in a data source system, how do we ensure that all the data loaded is valid?
  1. What can you tell me about mapplets?
  1. How would you store logs of previous sessions?
  1. What is a lookup transformation and how would you describe it?
  1. What is the difference between a router and a filter? When would you use either?
  1. What is active and passive transformation?
  1. What is the difference between static cache and dynamic cache?
  1. When can you use an SQL override during a lookup transformation?
  1. How can you delete duplicate rows and flat files?
  1. How many input parameters can there be in an unconnected lookup?
  1. What is parallel processing?
  1. How do you implement parallel processing?
  1. What are OLAP cubes?
  1. What is a Grain of Fact?
  1. What do you mean by data purging?
  1. What is tracing level and what are its types?
  1. Explain schema objects.
  1. Is sorter active or passive transformation?
  1. What is the difference between ETL and ELT?
  1. What is the difference between a shortcut and a reusable transformation?
  1. Describe an operational data store?
  1. Explain fact-less fact schema.
  1. What are dimensions?
  1. How would you explain ETL bugs?
  1. Explain a metadata extension.
  1. In your experience, have you ever dealt with Error Handling? Describe some of the techniques you used.

The ETL interview questions and answers will help you assess your candidate thoroughly. The way your candidate answers and the depth he goes into will show you the extent of his knowledge and let you better gauge whether he is, in fact, the perfect fit for your company. 

So since you already screen the resumes and put the candidates’ skills to the test before they appear for interviews, you get a very fine-tuned list of candidates left to interview. This means you can select and hire the perfect resource in very little time and with minimal effort like we promised.

FAQs for ETL recruiters

1. How do I prepare for an advanced ETL testing interview?

Answer: As an employer, you know that you need ETL testing to keep a check on your ETL development. Hence, the questions you ask here are slightly different. You have to make sure the candidate has a good basic knowledge of ETL but a very good command over SQL and scripting languages. Knowledge of a few ETL tools may be an advantage.

2. What is ETL scenario-based interview questions?

Answer: This is when you present a scenario to the candidate, and you ask them to answer questions keeping that scenario in mind. This tests on the spot problem-solving skills, since you can give them real-life scenarios and see if they respond in a way you expect them to or even better if they think out of the box and surprise you with their answer.

3. What are the three tiers in python ETL interview questions?

Answer: Data warehouses have three tiers. The first tier is the one where data is collected initially from various sources. The second tier is the integration tier. This is where the transformation of data takes place and data is made to fit the requirements of the company. The third tier is the dimensional tier and it’s the one where the formatted data is finally stored and available for use.