The answer is a row with the same primary key was already loaded in a previous batch. This is the most likely cause of a batch load primary key violation error because batch loads typically use INSERT operations, and relational databases enforce primary key uniqueness as a constraint; attempting to insert a duplicate key value triggers an immediate violation, halting the entire load process. On the CompTIA Data+ DA0-001 exam, this scenario tests your understanding of data integrity constraints and common ETL failure modes—a common trap is assuming the error comes from data type mismatches or null values, but the core issue is always duplicate keys from prior loads. Remember the memory tip: "PK = Previously Keyed"—if the primary key already exists, the batch load breaks.
DA0-001 Comparing and Contrasting Data Concepts Practice Question
This DA0-001 practice question tests your understanding of comparing and contrasting data concepts. Read the scenario carefully and evaluate each option against the stated constraints before committing to an answer. After answering, compare your reasoning against the explanation and wrong-answer breakdown below. Once you have made your selection, read the full explanation to reinforce the concept and understand why each distractor is designed to mislead on exam day.
Exhibit
Refer to the exhibit.
Error log from a data pipeline:
[2025-03-15 10:32:14] ERROR: Duplicate key value violates unique constraint 'order_pkey'
[2025-03-15 10:32:14] Detail: Key (order_id)=(12345) already exists.
[2025-03-15 10:32:15] WARNING: Batch load incomplete. 4999 of 5000 rows inserted.
A data analyst is reviewing the error log from a nightly batch load. What is the most likely cause of the error?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue: "most likely"
Why it matters: Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
A row with the same primary key was already loaded in a previous batch.
The error log from a nightly batch load indicates a primary key violation. This occurs when a row with the same primary key value already exists in the target table from a previous batch load. Since batch loads typically use INSERT operations, attempting to insert a duplicate primary key will raise a constraint violation error, halting the load process.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
✓
A row with the same primary key was already loaded in a previous batch.
Why this is correct
The error explicitly says duplicate key.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
✗
The data type of order_id is incorrect.
Why it's wrong here
No type conversion error.
✗
The source and target schemas are mismatched.
Why it's wrong here
No schema mismatch error is shown.
✗
The order_id field contains null values.
Why it's wrong here
No null violation error.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates confuse a primary key violation with a data type mismatch or schema mismatch, but the error log's specific reference to a duplicate key points directly to the primary key constraint.
Trap categories for this question
Command / output trap
No schema mismatch error is shown.
Detailed technical explanation
How to think about this question
Primary key constraints are enforced by a unique index (often a B-tree) on the primary key column(s). When a batch INSERT attempts to add a row with a key that already exists in the index, the database engine immediately raises an ORA-00001 (Oracle) or Violation of PRIMARY KEY constraint (SQL Server) error. In real-world ETL pipelines, this is commonly handled by using a MERGE (UPSERT) statement or by pre-checking for existing keys before the load.
KKey Concepts to Remember
Read the scenario before looking for a memorised answer.
Find the constraint that changes the correct option.
Eliminate answers that are true in general but not in this case.
TExam Day Tips
→Watch for words such as best, first, most likely and least administrative effort.
→Review why wrong options are wrong, not only why the correct option is correct.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
How this comes up in practice
A practitioner preparing for the DA0-001 exam encounters this exact type of scenario on the job. The correct answer here is not the most general option — it is the best answer for the specific constraint described. Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option. Real exam questions reward reading the full scenario before eliminating options, because the constraint defines which answer fits.
What to study next
Got this wrong? Here's your next step.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Comparing and Contrasting Data Concepts — This question tests Comparing and Contrasting Data Concepts — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: A row with the same primary key was already loaded in a previous batch. — The error log from a nightly batch load indicates a primary key violation. This occurs when a row with the same primary key value already exists in the target table from a previous batch load. Since batch loads typically use INSERT operations, attempting to insert a duplicate primary key will raise a constraint violation error, halting the load process.
What should I do if I get this DA0-001 question wrong?
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Are there clue words in this question I should notice?
Yes — watch for: "most likely". Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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Question Discussion
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