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Top ETL Interview Questions and Answers

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ETL or Extract-Transform-Load is the process of Extraction, Transformation, and Loading. In this process, ETL systems serve as the foundation of the data warehouse. The role of any ETL team involves building the back room of a data warehouse. ETL systems extract data from the source systems, implement data quality and consistency standards, confirm data, and deliver data in a presentable format.

The ETL teams need to deliver data effectively to end-user tools, document and protect data lineage, extract data from the source, and assure quality. They also need to maintain consistency across sources, deliver data in a format usable by query tools, clean, conform, add value to data, etc. In this article, we’ll look at some ETL interview questions to help you solidify your interview preparation.

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In this article, we’ll cover:

  • ETL and ETL Testing Process Interview Questions and Answers
  • Sample ETL Testing Interview Questions for Practice
  • ETL Developer Interview Questions
  • SQL Interview Questions for ETL Tester
  • FAQs on ETL Interview Questions

ETL and ETL Testing Process Interview Questions and Answers

Here are some ETL interview questions and answers to give you an idea of what to expect:

1. What is the significance of the ETL system in the Data warehouse?

The ETL system helps by removing mistakes, providing confidence measures in data, correcting missing data, tracking the flow of transnational data for safekeeping, presenting data such that end-user tools can use it, and adjusting data from multiple sources so they can be used together.

2. Define the grain of fact and mapping in ETL.

The lowest/most atomic level of data stored in the fact table by which we can define the facts is called the grain of fact or Fact Granularity. 

Mapping is the flow of data from the source to the destination.

3. Define a materialized view and a session in ETL.

A set of instructions describing the data movement from the source to the destination is called a session. A materialized view simply refers to an aggregate table.

4. Define an ETL validator and a Worklet in ETL.

A worklet refers to a set of tasks in ETL. It can refer to any set of tasks in the program.

An ETL Validator is a data testing tool to significantly simplify the testing of the Data Warehouse, Data Integration, and Data Migration projects.

5. What are some ETL test cases?

Common ETL test cases include Mapping Doc Validation, Data Quality, Correctness Issues, and Constraint Validation.

Sample ETL Testing Interview Questions for Practice

Check out these ETL testing interview questions to gauge your preparation level:

  1. Define and explain:
  • ETL
  • ETL mapping sheets
  • ETL process
  • Initial load
  • Full load
  • Snapshots
  • Views
  • Materialized view log 
  • Partitioning
  • Hash partitioning 
  • Round-robin partitioning
  1. Describe:
  • The steps involved in an ETL process
  • The three tiers in ETL
  1. What are the characteristics of snapshots?
  2. How can we fine-tune mapping in ETL?
  3. Differentiate between unconnected and connected lookup.
  4. How do we analyze tables in ETL?
  5. How is ETL testing used in third-party data management?

ETL Developer Interview Questions

Again, on the technical side, see if you can answer these ETL developer interview questions:

  1. Describe: 
  • The day-to-day process in the ETL System
  • The most common transformations in ETL processes
  • The importance of impact analysis in the ETL system
  • How to process the fixed-length flat file
  • How to prepare and develop incremental loads
  • Update a large table having over 10 million rows
  • Logging preparation for the ETL process
  • The most critical steps in data profiling
  • The three different approaches to implementing row versioning
  1. Define and Describe:
  • Staging area
  • incremental load
  • 3-tier system in ETL
  • Data Profiling

  1. Why do we need Real-Time ETL in the data warehouse?
  2. Why is a “staging” area needed?
  3. How are third-party tools like SSIS advantageous compared to SQL scripts?
  4. What is the use of data profiling in an ETL process? 
  5. How many steps are involved in an ETL process, and what are they?
  6. Are there sub-steps for each of the ETL steps?
  7. What are the three tiers in ETL?
  8. Names and describe the layers in ETL?

SQL Interview Questions for ETL Tester

Lastly, make sure you can answer these SQL interview questions before your interview:

  1. For ETL testing, how would you write an SQL query for the following:
  • Data Type Check
  • Data Length Check
  • Data Length Check Index / Constraint Check
  • Metadata Naming Standards Check
  • Metadata Check Across Environments
  1. Differentiate between:
  • ETL and ELT
  • Initial load and full load
  • ETL and SQL
  1. Is SQL an ETL?
  2. For better performance, what would you choose: Filter data first and then join it with other sources, or join it first and then filter?
  3. Describe the extraction phase in ETL in SQL?
  4. Describe the advantages and disadvantages associated with indexes?
  5. Is SQL Developer an ETL tool?
  6. How is SQL used in ETL?

FAQs on ETL Interview Questions

Q1. What is partitioning in ETL? What are the ETL partition types?

The sub-division of the transactions for enhancing their performance is called partitioning. Round-Robin partition and Hash partition are the two types of partitions in ETL.

Q2. What are the data formats available in the ETL system?

There are various data formats in ETL. They include flat files, XML datasets, dimensional data models, independent DBMS working tables, normalized entity/relationship (E/R) schemas.

Q3. What are some of the most used ETL tools?

Some of the most preferred ETL tools include Oracle Warehouse Builder, SAP Data Services, SAS Data Management, IBM Infosphere Information Server, Elixir Repertoire for Data ETL, Power Center Informatica, SQL Server Integration Services, and Data Migrator IBI.

Q4. What is the 3-tier system in ETL?

The 3-tier system in ETL refers to the data warehouse. Its middle tier gives the end-users securely usable data, and the other two layers are on either side of this middle tier. The other two layers are the end-users and the back-end data storage.

Q5. What are the operations involved in the ETL system?

There are three ETL Operations. The first step is extracting from a source, which can be a transnational system like Microsoft, Oracle, or any relational database server. In the second step, this data is then transformed via data cleansing operations to generate a business roll. Lastly, the data is loaded into the target database. 

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