This PCDE practice question tests your understanding of design and implement database schemas. 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.
```
gcloud spanner databases ddl describe my-database --instance=test-instance
---
CREATE TABLE Sensors (
SensorId INT64 NOT NULL,
Location STRING(100),
) PRIMARY KEY (SensorId);
CREATE TABLE Readings (
SensorId INT64 NOT NULL,
Timestamp TIMESTAMP NOT NULL OPTIONS (allow_commit_timestamp=true),
Value FLOAT64,
) PRIMARY KEY (SensorId, Timestamp DESC),
INTERLEAVE IN PARENT Sensors ON DELETE CASCADE;
```
Based on the exhibit, what is the primary key of the Readings table?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue: "primary"
Why it matters: Asks for the main purpose or function, not a secondary benefit. Eliminate answers that describe side-effects or partial functions.
Exhibit
Refer to the exhibit.
```
gcloud spanner databases ddl describe my-database --instance=test-instance
---
CREATE TABLE Sensors (
SensorId INT64 NOT NULL,
Location STRING(100),
) PRIMARY KEY (SensorId);
CREATE TABLE Readings (
SensorId INT64 NOT NULL,
Timestamp TIMESTAMP NOT NULL OPTIONS (allow_commit_timestamp=true),
Value FLOAT64,
) PRIMARY KEY (SensorId, Timestamp DESC),
INTERLEAVE IN PARENT Sensors ON DELETE CASCADE;
```
A
(SensorId, Timestamp)
The DDL explicitly defines this as the primary key.
B
(SensorId, SensorType)
Why wrong: SensorType is not defined in the table.
C
(SensorId)
Why wrong: The primary key includes both SensorId and Timestamp.
The answer is (SensorId, Timestamp). This is correct because the Cloud Spanner DDL explicitly defines the primary key as PRIMARY KEY (SensorId, Timestamp DESC), meaning the key is a composite of both columns, with Timestamp stored in descending order for efficient range scans on recent data. On the Google Professional Cloud Database Engineer exam, this tests your ability to read DDL syntax and distinguish between actual key columns and non-existent columns like SensorType or ReadingsId, which are common distractors. A frequent trap is assuming a single-column key or inventing an auto-generated ID column, but Spanner requires explicit key definition. Remember the memory tip: “Spanner keys are always explicit—if it’s not in the DDL’s PRIMARY KEY clause, it’s not part of the key.”
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
✓
(SensorId, Timestamp)
The Readings table captures sensor measurements over time, so the natural primary key is the combination of SensorId and Timestamp, which uniquely identifies each reading. This is a classic example of a composite primary key in relational database design, ensuring that no two readings from the same sensor at the same time can exist.
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.
✓
(SensorId, Timestamp)
Why this is correct
The DDL explicitly defines this as the primary key.
Clue confirmation
The clue word "primary" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
✗
(SensorId, SensorType)
Why it's wrong here
SensorType is not defined in the table.
✗
(SensorId)
Why it's wrong here
The primary key includes both SensorId and Timestamp.
✗
(ReadingsId)
Why it's wrong here
No ReadingsId column exists.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The Google Professional Data Engineer exam often tests the misconception that a single column like SensorId can serve as a primary key, when in fact the combination of SensorId and Timestamp is required to guarantee uniqueness in a time-series table.
Detailed technical explanation
How to think about this question
In database normalization, a composite primary key like (SensorId, Timestamp) enforces entity integrity at the row level and supports efficient indexing for time-series queries. Under the hood, most SQL databases implement this as a B-tree index on the composite key, which optimises range scans for a given sensor over a time interval. In real-world IoT systems, this pattern is common because sensor data is append-only and naturally ordered by time.
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 cloud solutions architect for a retail company is evaluating services for a new workload. The correct answer here reflects best practice for the specific scenario described — not a general cloud recommendation. Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option. Cloud exam questions reward reading the constraint carefully: the same technology can be right or wrong depending on the use case.
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.
Design and implement database schemas — This question tests Design and implement database schemas — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: (SensorId, Timestamp) — The Readings table captures sensor measurements over time, so the natural primary key is the combination of SensorId and Timestamp, which uniquely identifies each reading. This is a classic example of a composite primary key in relational database design, ensuring that no two readings from the same sensor at the same time can exist.
What should I do if I get this PCDE 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: "primary". Asks for the main purpose or function, not a secondary benefit. Eliminate answers that describe side-effects or partial functions.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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