Question 691 of 760
DP-203 Develop data processing Practice Question
You are designing a data processing solution using Azure Databricks with Delta Lake. The data is ingested from multiple sources and needs to be deduplicated based on a composite key (source_id, record_id). New data may have duplicates within the same batch. Which write mode and table property should you use to handle this efficiently?
⚠ Common exam trap
Microsoft often tests the misconception that 'overwrite' mode or table properties like 'autoOptimize' can handle deduplication, but the correct approach requires explicit deduplication logic (like MERGE) because Delta Lake does not enforce unique constraints natively.
Answer choices
Why each option matters
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Use 'append' mode and perform a MERGE operation after write to deduplicate.
Using 'append' mode writes all incoming data as new files, and then performing a MERGE operation (upsert) based on the composite key (source_id, record_id) allows you to efficiently deduplicate both within the batch and against existing data. This approach leverages Delta Lake's ACID transactions and avoids the cost of rewriting entire partitions, making it suitable for handling duplicates from multiple sources.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✓
Use 'append' mode and perform a MERGE operation after write to deduplicate.
Why this is correct
Append with a subsequent MERGE allows custom dedup logic on composite key.
- ✗
Use 'overwrite' mode and enable 'delta.autoOptimize.optimizeWrite' = true.
Why it's wrong here
Overwrite replaces the entire table, not suitable for incremental dedup.
- ✗
Use 'ignore' mode and set 'delta.autoCompact' = true.
Why it's wrong here
Ignore mode skips writing if file exists, not dedup within batch.
- ✗
Use 'error' mode and enable 'delta.merge.onSchemaMismatch' = true.
Why it's wrong here
Error mode fails on duplicates, not desired.
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Last reviewed: Jun 30, 2026
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