PDE Ingesting and Processing the Data Practice Question
You are building a BigQuery table that contains nested and repeated fields (e.g., order with line items). You need to write a query that counts the number of line items per order. Which TWO SQL functions/techniques can you use?
⚠ Common exam trap
Google often tests the distinction between functions that operate on arrays directly (like ARRAY_LENGTH) versus those that require row-level expansion (like UNNEST), and candidates may mistakenly choose window functions or STRUCT-based aggregation that do not directly count array elements.
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
✓
UNNEST with COUNT
UNNEST flattens the repeated line items array into individual rows, allowing COUNT to aggregate the number of line items per order. Option D is correct because ARRAY_LENGTH directly returns the number of elements in the repeated field array, which corresponds to the line item count.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
Window function ROW_NUMBER
Why it's wrong here
ROW_NUMBER assigns a sequential position within a window partition; it numbers rows rather than counting repeated elements, so it cannot produce a per-order line-item total. It is tempting because it operates over partitions, and it would be correct for ranking or deduplicating rows, not aggregating array length.
- ✓
UNNEST with COUNT
Why this is correct
UNNEST flattens the repeated line_items array into individual rows, letting COUNT aggregate them per order. This directly satisfies the stem's requirement to count line items within nested, repeated fields, since COUNT alone cannot traverse array elements without first unnesting them into a queryable row set.
- ✗
STRUCT with aggregation
Why it's wrong here
STRUCT merely groups fields into a record; it holds no counting semantics and cannot tally repeated line items per order. It is tempting because STRUCT defines the nested schema itself, and it would be the right choice when constructing or reshaping nested records, not when aggregating their element counts.
- ✓
ARRAY_LENGTH
Why this is correct
ARRAY_LENGTH returns the number of elements in an array, so applying it to the repeated line_items field yields the line item count per order directly. This satisfies the nested and repeated field requirement, since repeated fields are represented as arrays in BigQuery's standard SQL.
- ✗
SELECT * EXCEPT
Why it's wrong here
SELECT * EXCEPT projects columns while excluding named ones; it performs no aggregation and cannot count repeated line items. It is tempting because it handles wide nested schemas conveniently, and it would be correct when trimming unwanted top-level columns from output, not when computing per-order line-item totals.
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