Etl Interview Questions

46 real Etl questions asked in live technical interviews — each with a model answer. Updated weekly.

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Which is SCD type 2. What is its behavior?🟢 Easy · Conceptual · Asked 10×Explain source-to-target mapping.🟡 Medium · Conceptual · Asked 5×Which end-to-end data pipeline do you recommend for processing large datasets?🟡 Medium · Conceptual · Asked 3×Tell me about your recent project.🟢 Easy · Behavioral · Asked 2×What are the key dimensions of data quality and how do you ensure them in a data pipeline?🟡 Medium · Conceptual · Asked 2×What does that data pipeline do? What were its sources, transformations, and target? How did you use Snowflake for fact and dimension tables?🟡 Medium · Conceptual · Asked 1×We have an environment where we process up to 10 million transactions per day and need to migrate the data from our transactional database to an analytical database like Snowflake nightly. The data is flaky, and for about 10 days a year we see higher bandwidth and throughput. How would you design the data pipeline to handle this scenario?🔴 Hard · System Design · Asked 1×If there are no downstream steps in your pipeline, how would you handle the next layer?🟡 Medium · Conceptual · Asked 1×You have an upstream producer that recently added a new column to the dataset. How would you handle this to ensure your downstream job continues to run without breaking?🟡 Medium · Conceptual · Asked 1×Implement SCD Type 2 by closing the old row and inserting a new row for every address change, using effective dates to track history.🟡 Medium · Coding · Asked 1×In ETL pipeline design, what are the most critical aspects you focus on to track data lineage, and can you write an SQL query to implement a Slowly Changing Dimension (SCD) Type 2?🟡 Medium · Coding · Asked 1×Describe the path a single file takes from the moment it lands in a storage location to the moment it is ready for querying, including all intermediate steps that can be queried.🟡 Medium · Conceptual · Asked 1×If something fails in the middle of a data pipeline, how do you detect it and where do you start troubleshooting? What did you need to set up before the components could communicate with each other?🟡 Medium · Conceptual · Asked 1×In an ETL pipeline, what steps run first and which steps must wait for others to complete?🟡 Medium · Conceptual · Asked 1×A client has four source systems: two databases, a daily flat‑file drop, and a RESTful API. They want all data in one warehouse, refreshed before business opens with a dashboard. Nothing exists yet. How would you build it?🔴 Hard · System Design · Asked 1×If a day's data is loaded twice, how do you detect it and fix it? Additionally, if something was wrong in yesterday's data and you need to reload a month of data, what do you do?🟡 Medium · Debugging · Asked 1×If a day's data is loaded twice, how do you detect it and fix it?🟡 Medium · Debugging · Asked 1×What did you do when a batch load introduced bad rows?🟡 Medium · Conceptual · Asked 1×How did data get into your system?🟢 Easy · Conceptual · Asked 1×A daily load needs to insert rows that are new and update rows that already exist. How do you implement this?🟡 Medium · Conceptual · Asked 1×Suppose you have a 100‑200 GB CSV file stored on S3. How would you use AWS Glue and Spark to transform the data and load the final dataset into a target data warehouse (e.g., Redshift or Snowflake)?🟡 Medium · Conceptual · Asked 1×How would you design an hourly sync job pipeline that writes to a golden layer used by BI tools, and how would you handle network outages that can cause failures?🟡 Medium · System Design · Asked 1×Which tool or method do you use to load data into the silver layer from blob storage?🟢 Easy · Conceptual · Asked 1×Suppose your AWS Glue job takes a very long time (e.g., 20 minutes or more). How would you investigate and optimize its performance?🟡 Medium · Conceptual · Asked 1×What is the difference between the bronze, silver, and gold layers in a data lake architecture, and how do you handle data once it reaches the silver layer? Also, how do you consolidate multiple data sources into a single table?🟡 Medium · Conceptual · Asked 1×What is the purpose of the load_ts column in this query, and how does it differ from transaction_ts?🟢 Easy · Conceptual · Asked 1×How would you read and process this raw transaction log file in a Spark or Pandas ETL job?🟡 Medium · Conceptual · Asked 1×What tools do you use in your end-to-end production pipeline, and how do they fit together?🟡 Medium · Conceptual · Asked 1×In your project, you bring JSON data, run a cron job, and load it into S3. What transformations do you perform, and what difficulties do you face in this flow?🟡 Medium · Conceptual · Asked 1×Do you analyze trading indicators or chart patterns, such as VOLUMUS, OB, and FVG, in your work?🟢 Easy · Conceptual · Asked 1×How do you build an ETL pipeline that transforms data, stores it in Amazon S3, and queries it using Amazon Athena?🟡 Medium · Conceptual · Asked 1×What are the best practices for designing a final target table in a data pipeline, especially when integrating with dashboards?🟡 Medium · Conceptual · Asked 1×In the recent project you worked on, how did you handle data ingestion from multiple source systems into your data warehouse?🟡 Medium · Behavioral · Asked 1×What are the key aspects to consider during an ETL code review?🟡 Medium · Conceptual · Asked 1×When reprocessing new jobs with stored procedures, what type of architecture would you use to handle a terabyte of data?🔴 Hard · System Design · Asked 1×Source data is in the bronze layer. We need to implement SCD type 2 in the target layer. If data arrives daily in batch and the bronze layer is refreshed each day, what logic would you use between the bronze and target layers to achieve SCD type 2?🟡 Medium · Conceptual · Asked 1×How would you implement logic for three ordered tiers (Bronze, Silver, Gold) in a target table?🟡 Medium · Conceptual · Asked 1×Which tools could you use for bringing all data out of a data lake?🟡 Medium · Conceptual · Asked 1×Can you describe a project where you had to ingest advertising data from multiple platforms (e.g., TM360, DB360, TradeDesk) and handle data quality issues in custom offline data?🟡 Medium · Behavioral · Asked 1×In a previous design, a view was built to directly query data from S3, causing slowdown. How did you address this performance issue?🟡 Medium · Debugging · Asked 1×What is AWS Glue about? Why use it? What are its advantages?🟡 Medium · Conceptual · Asked 1×What type of resource were you using to transform raw files from bronze to silver to gold layers?🟡 Medium · Conceptual · Asked 1×Mention the steps you took to move data from source to the bronze layer.🟡 Medium · Conceptual · Asked 1×How do you move data from one service to another?🟡 Medium · Conceptual · Asked 1×What techniques do you use to move data from one service to another, such as from X3 to Redshift?🟡 Medium · Conceptual · Asked 1×How long does it take to create an ETL pipeline to ingest data?🟡 Medium · System Design · Asked 1×

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