The correct choice is to use a foreign key constraint from Orders.customer_id to Customers.customer_id because this enforces referential integrity directly at the database level, ensuring that every order references an existing customer without relying on error-prone application logic. Cloud SQL natively supports foreign key constraints in both MySQL and PostgreSQL, and for high write throughput, the database handles the check atomically within the same transaction, which prevents race conditions and maintains data consistency. On the Google Professional Cloud Database Engineer exam, this scenario tests your understanding of when to use declarative constraints versus application-level enforcement; a common trap is assuming that application logic is safer for performance, but database-level constraints are actually more efficient for consistency under concurrent writes. Remember the mnemonic “FK for Integrity” — foreign keys are the database’s built-in guarantee that your relationships stay valid, no matter how fast the writes come.
PCDE Design and implement database schemas Practice Question
This PCDE practice question tests your understanding of design and implement database schemas. Compare every option against the stated constraints before choosing — the best answer satisfies all requirements, not just the most obvious one. 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.
A team is designing a relational schema for a new application on Cloud SQL. The schema includes a table 'Orders' and a table 'Customers'. Each order belongs to one customer. The team anticipates high write throughput and needs to enforce referential integrity. Which schema design is most appropriate?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Use a foreign key constraint from Orders.customer_id to Customers.customer_id
Option D is correct because using a foreign key constraint from Orders.customer_id to Customers.customer_id enforces referential integrity at the database level, which is essential for maintaining data consistency in a relational schema. Cloud SQL (e.g., MySQL or PostgreSQL) natively supports foreign key constraints, ensuring that every order references an existing customer without relying on application logic. This approach is efficient for high write throughput as the database handles the check atomically, avoiding race conditions.
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.
✗
Use Cloud Spanner interleaved tables with Orders as a child of Customers
Why it's wrong here
Interleaved tables do not enforce foreign key constraints across non-interleaved tables.
✗
Implement referential integrity checks in the application code and omit database constraints
Why it's wrong here
Risk of data inconsistency due to concurrent writes.
✗
Store order data as a JSON array in a column of the Customers table
Why it's wrong here
Denormalization leads to update anomalies and poor query performance.
✓
Use a foreign key constraint from Orders.customer_id to Customers.customer_id
Why this is correct
Enforces integrity efficiently within the database.
Related concept
Read the scenario before looking for a memorised answer.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Google Cloud often tests the misconception that application-level checks are sufficient for high-throughput systems, but the trap here is that database-level foreign keys are the only way to guarantee referential integrity under concurrent writes, as application code cannot prevent race conditions or orphaned records.
Detailed technical explanation
How to think about this question
Foreign key constraints in Cloud SQL (e.g., InnoDB engine) use index-backed checks to validate referential integrity with minimal overhead, even under high write loads. The database locks the parent row during the insert to prevent race conditions, ensuring ACID compliance. In practice, this design also supports cascading operations (e.g., ON DELETE CASCADE) to maintain consistency without application-level logic.
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: Use a foreign key constraint from Orders.customer_id to Customers.customer_id — Option D is correct because using a foreign key constraint from Orders.customer_id to Customers.customer_id enforces referential integrity at the database level, which is essential for maintaining data consistency in a relational schema. Cloud SQL (e.g., MySQL or PostgreSQL) natively supports foreign key constraints, ensuring that every order references an existing customer without relying on application logic. This approach is efficient for high write throughput as the database handles the check atomically, avoiding race conditions.
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.
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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