ETL/ELT interview questions

24 ETL/ELT questions from the Data Engineering bank, written for Indian campus drives and tech interviews. Every question has a verified answer and an AI-tutor explanation on placd.

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1. What is ETL?

Junior
  1. A.reprocessing historical data, safe only when writes are idempotent
  2. B.re-running a job with the same input yields the same result, with no duplicates
  3. C.transforming data before loading it into the target
  4. D.overwriting a target partition rather than appending

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2. Which term means: "transforming data before loading it into the target"?

Junior
  1. A.idempotent write
  2. B.MERGE / upsert
  3. C.idempotency
  4. D.ETL

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3. Which statement is correct?

Junior
  1. A.ETL — transforming data before loading it into the target
  2. B.ETL — capturing changes from the database transaction log (e.g. Debezium), including deletes
  3. C.ETL — processing only new or changed rows since the last run
  4. D.ETL — updating matched rows and inserting new rows in one atomic statement

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4. What is ELT?

Junior
  1. A.overwriting a target partition rather than appending
  2. B.reprocessing historical data, safe only when writes are idempotent
  3. C.capturing changes from the database transaction log (e.g. Debezium), including deletes
  4. D.loading raw data first, then transforming it inside the warehouse

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5. Which term means: "loading raw data first, then transforming it inside the warehouse"?

Junior
  1. A.ELT
  2. B.idempotent write
  3. C.log-based CDC
  4. D.ETL

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6. Which statement is correct?

Junior
  1. A.ELT — loading raw data first, then transforming it inside the warehouse
  2. B.ELT — reprocessing historical data, safe only when writes are idempotent
  3. C.ELT — overwriting a target partition rather than appending
  4. D.ELT — transforming data before loading it into the target

Answer + AI explanation with Pro

7. What is idempotency?

Mid
  1. A.re-running a job with the same input yields the same result, with no duplicates
  2. B.loading raw data first, then transforming it inside the warehouse
  3. C.capturing changes from the database transaction log (e.g. Debezium), including deletes
  4. D.overwriting a target partition rather than appending

Answer + AI explanation with Pro

8. Which term means: "re-running a job with the same input yields the same result, with no duplicates"?

Mid
  1. A.MERGE / upsert
  2. B.idempotency
  3. C.incremental load
  4. D.idempotent write

Answer + AI explanation with Pro

9. Which statement is correct?

Mid
  1. A.idempotency — re-running a job with the same input yields the same result, with no duplicates
  2. B.idempotency — transforming data before loading it into the target
  3. C.idempotency — processing only new or changed rows since the last run
  4. D.idempotency — capturing changes from the database transaction log (e.g. Debezium), including deletes

Answer + AI explanation with Pro

10. What is idempotent write?

Mid
  1. A.transforming data before loading it into the target
  2. B.capturing changes from the database transaction log (e.g. Debezium), including deletes
  3. C.overwriting a target partition rather than appending
  4. D.processing only new or changed rows since the last run

Answer + AI explanation with Pro

11. Which term means: "overwriting a target partition rather than appending"?

Mid
  1. A.idempotency
  2. B.incremental load
  3. C.ELT
  4. D.idempotent write

Answer + AI explanation with Pro

12. Which statement is correct?

Mid
  1. A.idempotent write — updating matched rows and inserting new rows in one atomic statement
  2. B.idempotent write — re-running a job with the same input yields the same result, with no duplicates
  3. C.idempotent write — overwriting a target partition rather than appending
  4. D.idempotent write — processing only new or changed rows since the last run

Answer + AI explanation with Pro

13. What is incremental load?

Mid
  1. A.capturing changes from the database transaction log (e.g. Debezium), including deletes
  2. B.reprocessing historical data, safe only when writes are idempotent
  3. C.processing only new or changed rows since the last run
  4. D.loading raw data first, then transforming it inside the warehouse

Answer + AI explanation with Pro

14. Which term means: "processing only new or changed rows since the last run"?

Mid
  1. A.ELT
  2. B.incremental load
  3. C.idempotent write
  4. D.ETL

Answer + AI explanation with Pro

15. Which statement is correct?

Mid
  1. A.incremental load — processing only new or changed rows since the last run
  2. B.incremental load — capturing changes from the database transaction log (e.g. Debezium), including deletes
  3. C.incremental load — updating matched rows and inserting new rows in one atomic statement
  4. D.incremental load — transforming data before loading it into the target

Answer + AI explanation with Pro

16. What is MERGE / upsert?

Mid
  1. A.transforming data before loading it into the target
  2. B.loading raw data first, then transforming it inside the warehouse
  3. C.updating matched rows and inserting new rows in one atomic statement
  4. D.processing only new or changed rows since the last run

Answer + AI explanation with Pro

17. Which term means: "updating matched rows and inserting new rows in one atomic statement"?

Mid
  1. A.idempotency
  2. B.ETL
  3. C.incremental load
  4. D.MERGE / upsert

Answer + AI explanation with Pro

18. Which statement is correct?

Mid
  1. A.MERGE / upsert — updating matched rows and inserting new rows in one atomic statement
  2. B.MERGE / upsert — processing only new or changed rows since the last run
  3. C.MERGE / upsert — overwriting a target partition rather than appending
  4. D.MERGE / upsert — reprocessing historical data, safe only when writes are idempotent

Answer + AI explanation with Pro

19. What is log-based CDC?

Senior
  1. A.loading raw data first, then transforming it inside the warehouse
  2. B.capturing changes from the database transaction log (e.g. Debezium), including deletes
  3. C.transforming data before loading it into the target
  4. D.reprocessing historical data, safe only when writes are idempotent

Answer + AI explanation with Pro

20. Which term means: "capturing changes from the database transaction log (e.g. Debezium), including deletes"?

Senior
  1. A.idempotent write
  2. B.log-based CDC
  3. C.ETL
  4. D.MERGE / upsert

Answer + AI explanation with Pro

21. Which statement is correct?

Senior
  1. A.log-based CDC — overwriting a target partition rather than appending
  2. B.log-based CDC — capturing changes from the database transaction log (e.g. Debezium), including deletes
  3. C.log-based CDC — re-running a job with the same input yields the same result, with no duplicates
  4. D.log-based CDC — transforming data before loading it into the target

Answer + AI explanation with Pro

22. What is backfill?

Mid
  1. A.transforming data before loading it into the target
  2. B.reprocessing historical data, safe only when writes are idempotent
  3. C.loading raw data first, then transforming it inside the warehouse
  4. D.processing only new or changed rows since the last run

Answer + AI explanation with Pro

23. Which term means: "reprocessing historical data, safe only when writes are idempotent"?

Mid
  1. A.backfill
  2. B.log-based CDC
  3. C.MERGE / upsert
  4. D.idempotency

Answer + AI explanation with Pro

24. Which statement is correct?

Mid
  1. A.backfill — updating matched rows and inserting new rows in one atomic statement
  2. B.backfill — loading raw data first, then transforming it inside the warehouse
  3. C.backfill — processing only new or changed rows since the last run
  4. D.backfill — reprocessing historical data, safe only when writes are idempotent

Answer + AI explanation with Pro

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