DA0-002 Data Acquisition and Preparation Practice Question
A marketing analyst is combining two datasets: one containing campaign IDs and spend, and another containing campaign IDs and impressions. The first dataset has 1,200 rows and the second has 950 rows. After an inner join on campaign_id, the result has 1,050 rows. Which statement best explains this result?
⚠ Common exam trap
The trap here is assuming any row count change during a join indicates an error, when an inner join legitimately reduces rows to the matching key intersection.
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
✓
The inner join retained only rows where campaign_id exists in both datasets, so campaigns present in only one dataset were excluded.
An inner join returns only rows whose join key appears in both inputs. Given 1,200 and 950 source rows, an output of 1,050 indicates substantial but incomplete overlap, which is exactly what an inner join produces. A Cartesian product, cross join, or referential integrity enforcement would not yield this count. The analyst observed normal inner join behavior reflecting partial key matching between the two campaign datasets.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✓
The inner join retained only rows where campaign_id exists in both datasets, so campaigns present in only one dataset were excluded.
Why this is correct
An inner join returns only the intersection of keys from both tables. With 1,200 and 950 rows, the result of 1,050 means many campaigns matched but some from each side did not. This is the expected behavior of an inner join and explains why the output is smaller than the larger input while still substantial, reflecting partial overlap between the two campaign lists.
- ✗
The inner join produced a Cartesian product because campaign_id was not unique in one of the datasets.
Why it's wrong here
A Cartesian product would produce a row count equal to the product of the two table sizes, far larger than either input, not 1,050. While duplicate campaign_id values can inflate results, the described count is smaller than the larger input, which is inconsistent with a Cartesian explosion. The more likely explanation is simply that only matching keys were retained by the inner join.
- ✗
The result indicates referential integrity was enforced, so orphaned campaign IDs were automatically repaired.
Why it's wrong here
Referential integrity constraints prevent inserts or updates that would create orphaned keys, but they do not repair or reconcile data during a join. The row count of 1,050 is explained by the inner join's intersection semantics, not by constraint enforcement. Referential integrity also would not change the number of matching rows returned by a query.
- ✗
The join failed to include unmatched rows because the analyst should have used a cross join to preserve all campaigns.
Why it's wrong here
A cross join pairs every row of one table with every row of the other, producing 1,140,000 rows here, which is clearly not the observed result and would not preserve meaningful matches. Cross joins are used for generating combinations, not for combining related datasets on a key. The analyst's inner join behaved correctly given the intent to combine spend and impressions by campaign.
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Last reviewed September 2026 · checked against the official CompTIA exam blueprint
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